The Mosque–Cathedral of Córdoba (Spanish: Mezquita-Catedral de Córdoba [meθˈkita kateˈðɾal de ˈkoɾðoβa]), officially known by its ecclesiastical name of Cathedral of Our Lady of the Assumption (Spanish: Catedral de Nuestra Señora de la Asunción), is the cathedral of the Diocese of Córdoba dedicated to the Assumption of Mary and located in the Spanish region of Andalusia. Due to its status as a former mosque, it is also known in Spanish as the Mezquita (Spanish for 'mosque') and in an historical sense as the Great Mosque of Córdoba.

According to traditional accounts a Visigothic church, the Catholic Christian Basilica of Vincent of Saragossa, originally stood on the site of the current Mosque-Cathedral, although this has been a matter of scholarly debate. The Great Mosque was constructed in 785 on the orders of Abd al-Rahman I, founder of the Islamic Emirate of Córdoba. It was expanded multiple times afterwards under Abd al-Rahman's successors up to the late 10th century. Among the most notable additions, Abd al-Rahman III added a minaret (finished in 958) and his son al-Hakam II added a richly-decorated new mihrab and maqsurah section (finished in 971). The mosque was converted to a cathedral in 1236 when Córdoba was captured by the Christian forces of Castile during the Reconquista. The structure itself underwent only minor modifications until a major building project in the 16th century inserted a new Renaissance cathedral nave and transept into the center of the building. The former minaret, which had been converted to a bell tower, was also significantly remodelled around this time. Starting in the 19th century, modern restorations have in turn led to the recovery and study of some of the building's Islamic-era elements. Today, the building continues to serve as the city's cathedral and Mass is celebrated there daily.

The mosque structure is an important monument in the history of Islamic architecture and was highly influential on the subsequent "Moorish" architecture of the western Mediterranean regions of the Muslim world. It is also one of Spain's major historic monuments and tourist attractions, as well as a UNESCO World Heritage Site since 1984.

A claim that the site of the mosque-cathedral was once a Roman temple dedicated to Janus dates as far back as Pablo de Céspedes and is sometimes still repeated today. However, Robert Knapp, in his overview of Roman-era Córdoba, has dismissed this claim as speculation based on a misunderstanding of Roman milestones found in the area.

According to traditional accounts, the present-day site of the Cathedral–Mosque of Córdoba was originally a Visigothic Christian church dedicated to Saint Vincent of Saragossa, which was divided and shared by Christians and Muslims after the Umayyad conquest of Hispania. As the Muslim community grew and this existing space became too small for prayer, the basilica was expanded little by little through piecemeal additions to the building. This sharing arrangement of the site lasted until 785, when the Christian half was purchased by Abd al-Rahman I, who then proceeded to demolish the church structure and build the grand mosque of Córdoba on its site. In return, Abd al-Rahman also allowed the Christians to rebuild other ruined churches – including churches dedicated to the Christian martyrs Saints Faustus, Januarius, and Marcellus whom they deeply revered – as agreed upon in the sale terms.

The historicity of this narrative has been challenged as archaeological evidence is scant and the narrative is not corroborated by contemporary accounts of the events following Abd al-Rahman I's initial arrival in al-Andalus. The narrative of the church being transformed into a mosque, which goes back to the tenth-century historian Al-Razi, echoed similar narratives of the Islamic conquest of Syria, in particular the story of building the Umayyad Mosque in Damascus. For medieval Muslim historians, these parallels served to highlight a dynastic Umayyad conquest of Spain and appropriation of Visigothic Córdoba. Another tenth-century source mentions a church that stood at the site of the mosque without giving further details. An archaeological exhibit in the mosque–cathedral of Cordoba today displays fragments of a Late Roman or Visigothic building, emphasizing an originally Christian nature of the complex. The "stratigraphy" of the site is complicated and made more so by its impact on contemporary political debates about cultural identity in Spain.

According to Susana Calvo Capilla, a specialist on the history of the mosque–cathedral, although remains of multiple church-like buildings have been located on the territory of the mosque–cathedral complex, no clear archaeological evidence has been found of where either the church of St. Vincent or the first mosque were located on the site, and the latter may have been a newly constructed building. The evidence suggests that it may have been the grounds of an episcopal complex rather than a particular church which were initially divided between Muslims and Christians. Pedro Marfil, an archeologist at the University of Cordoba, has argued for the existence of such a complex – including a Christian basilica – on this site by interpreting the existing archeological remains. D. Fairchild Ruggles, a scholar of Islamic art, considers previous archeological work to be a confirmation of the former church's existence. This theory has been opposed by Fernando Arce-Sainz, another archeologist, who states that none of the numerous archeological investigations in modern times have turned up remains of Christian iconography, a cemetery, or other evidence that would support the existence of a church. Art historian Rose Walker, in an overview of late antique and early medieval art in Spain, has likewise criticized Marfil's view as relying on personal interpretation. More recently, archeologists Alberto León and Raimundo Ortiz Urbano have affirmed the hypothesis of a large episcopal complex by analyzing both old and new archeological findings at the site, while María de los Ángeles Utrero Agudo and Alejandro Villa del Castillo argue that evidence so far does not allow for the identification of former ecclesiastical structures on the site.

Regardless of what structures may have existed on the site, however, it is almost certain that the building which housed the city's first mosque was destroyed to build Abd ar-Rahman I's Great Mosque and that it had little relation to the latter's form.

The Great Mosque was built in the context of the new Umayyad Emirate in Al-Andalus which Abd ar-Rahman I founded in 756. Abd ar-Rahman was a fugitive and one of the last remaining members of the Umayyad royal family which had previously ruled the first hereditary caliphate based in Damascus, Syria. This Umayyad Caliphate was overthrown during the Abbasid Revolution in 750 and the ruling family were nearly all killed or executed in the process. Abd ar-Rahman survived by fleeing to North Africa and, after securing political and military support, took control of the Muslim administration in the Iberian Peninsula from its governor, Yusuf ibn Abd al-Rahman al-Fihri. Cordoba was already the capital of the Muslim province and Abd ar-Rahman continued to use it as the capital of his independent emirate.

Construction of the mosque began in 785–786 (169 AH) and finished a year later in 786–787 (170 AH). This relatively short period of construction was aided by the reuse of existing Roman and Visigothic materials in the area, especially columns and capitals. Syrian (Umayyad), Visigothic, and Roman influences have been noted in the building's design, but the architect is not known. The craftsmen working on the project probably included local Iberians as well as people of Syrian origin. According to tradition and historical written sources, Abd ar-Rahman involved himself personally and heavily in the project, but the extent of his personal influence in the mosque's design is debated.

The original mosque had a roughly square floor plan measuring 74 or 79 square meters per side, equally divided between a hypostyle prayer hall to the south and an open courtyard (sahn) to the north. As the mosque was built on a sloping site, a large amount of fill would have been necessary to create a level ground on which to build. The outer walls were reinforced with large buttresses, which are still visible on the exterior today.The original mosque's most famous architectural innovation, which was preserved and repeated in all subsequent Muslim-era expansions, was its rows of two-tiered arches in the hypostyle hall.

The mosque's original mihrab (niche in the far wall symbolizing the direction of prayer) no longer exists today but its probable remains were found during archeological excavations between 1932 and 1936. The remains showed that the mihrab's upper part was covered with a shell-shaped hood similar to the later mihrab.

The mosque originally had four entrances: one was in the center of the north wall of the courtyard (aligned with the mihrab to the south), two more were in the west and east walls of the courtyard, and a fourth one was in the middle of the west wall of the prayer hall. The latter was known as Bab al-Wuzara' (the "Viziers' Gate", today known as Puerta de San Esteban) and was most likely the entrance used by the emir and state officials who worked in the palace directly across the street from here.

The courtyard of the mosque was planted with trees as early as the 9th century, according to written sources cited by the 11th century jurist Ibn Sahl. Although the species of tree is not known, the fact that these were fruit trees is attested in Ibn Sahl, who was consulted as to whether such a garden was forbidden and, if not forbidden, whether it was permitted to eat from it. That the trees remained in the courtyard is demonstrated by two seals of the City of Cordoba, one in 1262 and the other in 1445, both of which show the mosque (which by then had been converted to a cathedral) within whose walls appear tall palm trees. This evidences makes the Cordoba mosque the earliest one where trees are known to have been planted in the courtyard.

Mosques were normally aligned with the qibla (the direction of prayer), which is theoretically the direction of Mecca. From Cordoba, Mecca is to the east-southeast, but the Great Mosque of Cordoba is instead oriented more towards the south. This orientation, which doesn't match that of modern mosques, reflects the pre-existing street alignment of Roman Cordoba.

It is also due to historical differences in opinion about the appropriate direction of the qibla in far western Islamic lands like al-Andalus and Morocco. In this early period, many Muslims in the region preferred a tradition that existed in the western Islamic world (the Maghreb and al-Andalus) according to which the qibla should be oriented towards the south instead of pointing towards the shortest distance to Mecca. This was based on a saying (hadith) of Muhammad which stated that "What is between the east and west is a qibla", which thus legitimized southern alignments.

This practice may also have sought to emulate the orientation of the walls of the rectangular Kaaba building inside the Great Mosque of Mecca, based on another tradition which considered the different sides of the Kaaba as being associated with different parts of the Muslim world. In this tradition the northwest face of the Kaaba was associated with al-Andalus and, accordingly, the Great Mosque of Cordoba was oriented towards the southeast as if facing the Kaaba's northwestern façade, with its main axis parallel to the main axis of the Kaaba structure (which was oriented from southeast to northwest).

Although later mosques in Al-Andalus did have more eastern-facing orientations (e.g. the Mosque of Madinat al-Zahra in the 10th century), later expansions of the Great Mosque did not attempt to modify its original alignment.

In 793 Abd ar-Rahman I's son and successor, Hisham I, added to the mosque a ṣawma'a, a shelter for the muezzin on top of the outer wall, as the mosque did not yet have a minaret (a feature which was not yet standard in early mosques).

The mosque was significantly expanded by Abd ar-Rahman II (r. 822–852) sometime between the years 833 and 848. This expansion preserved and repeated the original design while extending the prayer hall eight bays to the south (i.e. the length of eight arches). This made the prayer hall 64 metres long from front to back.

During this expansion, the builders began to commission new marble capitals for the columns instead of just re-using ancient ones. These new capitals were imitations of the Corinthian style but still differed slightly from classical models, thus hinting at the future evolution of architectural sculpture in al-Andalus. One probable example of these capitals is now preserved at the National Archaeological Museum in Madrid and features an Arabic inscription in an archaic Kufic script that offers blessings on Abd ar-Rahman II. The expansion work may have been unfinished when Abd ar-Rahman II died in 852 and it appears to have been completed instead by his son and successor, Muhammad I (r. 852–886).

Muhammad I carried out various other works on the mosque and is reported to have created a maqsura (a prayer space reserved for the ruler). In 855 he also restored the Bab al-Wuzara' gate (today's Puerta de San Esteban). The decoration of this gate, which thus likely dates from this time, is often noted as an important prototype of later Moorish gateways. Muhammad's son, Al-Mundhir (r. 886–888), in turn added a treasury to the mosque. Al-Mundhir's son, Abdallah (r. 888–912), built the mosque's first elevated passage, known as a sabat, which connected the mosque directly with the Umayyad palace across the street. This passage allowed the ruler thenceforth to enter the mosque privately, where he would remain unseen behind the screen of the maqsura, thus separating him from the general public during prayer. New versions of this bridge would later be rebuilt during the mosque's subsequent expansions.

In the 10th century Abd ar-Rahman III (r. 912–961) declared a new Caliphate in al-Andalus and inaugurated the height of Andalusi power in the region. As part of his various construction projects, he reworked and enlarged the courtyard of the Great Mosque and built its first true minaret (a tower from which the call to prayer was issued) starting in 951–952. The new works, including the minaret, were completed in 958, as recorded by a surviving inscription on a marble plaque that includes the name of Abd ar-Rahman III as well as the names of the master builder and the supervisor of works.

The minaret was 47 meters high and had a square base measuring 8.5 meters per side. Scholar Jonathan Bloom suggests that Abd ar-Rahman III's construction of the minaret – along with his sponsoring of other minarets around the same time in Fez, Morocco – was partly intended as a visual symbol of his growing authority as caliph and may have also been aimed at defying the rival Fatimid Caliphate to the east, which eschewed such structures.

Abd ar-Rahman III also reinforced the northern wall of the courtyard by adding another "façade" in front of the old one on the courtyard side. Historical accounts differ on whether the completed courtyard had a surrounding gallery or portico (as seen today and as was common in the courtyards of other mosques). Many modern scholars affirm that the courtyard was provided with an enveloping gallery at this time and that its design involved an alternation between piers and columns (similar to its current appearance).

Abd ar-Rahman III's son and successor, Al-Hakam II (r. 961–976), was a cultured man who was involved in his father's architectural projects. During his own reign, starting in 961, he further expanded the mosque's prayer hall. The hall was extended 45 meters to the south by adding 12 more bays (arches), again repeating the two-tiered arches of the original design. This expansion is responsible for some of the mosque's most significant architectural flourishes and innovations.

At the beginning of al-Hakam's extension, the central "nave" of the mosque was highlighted with an elaborate ribbed dome (now part of the Capilla da Villaviciosa). More famously, a rectangular maqsura area around the mosque's new mihrab was distinguished by a set of unique interlacing multifoil arches. The rectangular area within this, in front of the mihrab, was covered by three more decorative ribbed domes. The domes and the new mihrab niche were finished in November or December 965. An inscription records the names of four of its craftsmen, who also worked at the Reception Hall (Salon Rico) of Madinat al-Zahra.

Soon after this date both the middle dome of the maqsura and the wall surfaces around the mihrab were covered in rich Byzantine-influenced gold mosaics. According to traditional accounts like that of Ibn 'Idhari, Al-Hakam II had written to the Byzantine emperor (initially Nikephoros II Phokas) in Constantinople requesting that he send him expert mosaicists for the task. The emperor consented and sent him a master craftsman along with about 1600 kg of mosaic tesserae as a gift. The mosaicist trained some of the caliph's own craftsmen, who eventually became skilled enough to do the work on their own. The work was finished by this team in late 970 or early 971.

Al-Hakam II's work on the mosque also included the commissioning of a new minbar (pulpit) in 965, which took about 5 to 7 years to finish. Unfortunately, the details of its construction and of its chronology are muddled by sometimes contradictory historical sources. Ibn 'Idhari, for example, implies that Al-Hakam had two minbars built in this period, with one of them possibly having been destroyed or replaced. Either way, whichever minbar survived and became associated with the mosque was celebrated by many writers for its craftsmanship. It was made out of precious woods like ebony, boxwood, and "scented" woods, and it was inlaid with ivory and with other coloured woods such as red and yellow sandalwood. Modern scholars believe the minbar had wheels which allowed it to be rolled in and out of its storage chamber.

The mosque's last significant expansion under Muslim rule was ordered by Al-Mansur (Almanzor), the autocratic vizier of Caliph Hisham II, in 987–988. Rather than extending the mosque further south, which would have been impossible due to the proximity of the riverbank, Al-Mansur had the mosque extended laterally towards the east, extending both the courtyard and the prayer hall by 47.76 meters and adding eight naves to the mosque. The new extension covered 8600 square meters and made the mosque the largest in the Muslim world outside of Abbasid Iraq.

Once again, the same design of two-tiered arches was replicated in the new construction. However, the capitals produced for the hundreds of new columns have a simpler and less detailed design that may reflect the hurry in which they were produced.The new eastern wall of the mosque featured ten richly-decorated exterior portals similar to the ones on the mosque's western side, although these were heavily restored in the 20th century. Al-Mansur also famously looted the bells of the Cathedral of Santiago de Compostela and reportedly ordered them to be melted down and turned into chandeliers for the mosque, although none of these chandeliers have survived.

After the collapse of the Umayyad Caliphate in Cordoba at the beginning of the 11th century, no further expansions to the mosque were carried out. Indeed, the collapse of authority had immediate negative consequences for the mosque, which was looted and damaged during the fitna (civil conflict) that followed the caliphate's fall (roughly between 1009 and 1030). Cordoba itself also suffered a decline but remained an important cultural center. Under Almoravid rule, the artisan workshops of Cordoba were commissioned to design new richly-crafted minbars for the most important mosques of Morocco – most famously the Minbar of the Kutubiyya Mosque commissioned in 1137 – which were likely inspired by the model of al-Hakam II's minbar in the Great Mosque.

In 1146 the Christian army of King Alfonso VII of León and Castile briefly occupied Cordoba. The archbishop of Toledo, Raymond de Sauvetât, accompanied by the king, led a mass inside the mosque to "consecrate" the building. According to Muslim sources, before leaving the city the Christians plundered the mosque, carrying off its chandeliers, the gold and silver finial of the minaret, and parts of the rich minbar. As a result of both this pillage and the earlier pillage during the fitna, the mosque had lost almost all of its valuable furnishings.

In 1162, after a general period of decline and recurring sieges, the Almohad caliph Abd al-Mu'min ordered that Cordoba be prepared to become his capital in al-Andalus. As part of this preparation, his two sons and governors, Abu Yaqub Yusuf and Abu Sa'id, ordered that the city and its monuments be restored. The architect Ahmad ibn Baso (who was later known for his work on the Great Mosque of Seville) was responsible for carrying out this restoration program. It is not known exactly which buildings he restored, but it is almost certain that he restored the Great Mosque. It is likely that the mosque's minbar was also restored at this time, since it is known to have survived long afterwards up to the 16th century.

In 1236 Córdoba was conquered by King Ferdinand III of Castile as part of the Reconquista. Upon the city's conquest the mosque was converted into a Catholic cathedral dedicated to the Virgin Mary (Santa Maria). The first mass was dedicated here on June 29 of that year. According to Jiménez de Rada, Ferdinand III also carried out the symbolic act of returning the former cathedral bells of Santiago de Compostela that were looted by Al-Mansur (and which had been turned into mosque lamps) back to Santiago de Compostela.

Despite the conversion, the early Christian history of the building saw only minor alterations being done to its structure, mostly limited to the creation of small chapels and the addition of new Christian tombs and furnishings. Even the mosque's minbar was apparently preserved in its original storage chamber, though it is unknown if it was used in any way during this time. (The minbar has since disappeared, but it still existed in the 16th century, when it was apparently seen by Ambrosio de Morales.)

The cathedral's first altar was installed in 1236 under the large ribbed dome at the edge of Al-Hakam II's 10th-century extension of the mosque, becoming part of what is today called the Villaviciosa Chapel (Capilla de Villaviciosa) and the cathedral's first main chapel (the Antigua Capilla Mayor). There is no indication that even this space was significantly modified in its structure at this time. The area of the mosque's mihrab and maqsura, along the south wall, was converted into the Chapel of San Pedro and was reportedly where the host was stored. What is today the 17th-century Chapel of the Conception (Capilla de Nuestra Señora de la Concepción), located on the west wall near the courtyard, was initially the baptistery in the 13th century. These three areas appear to have been the most important focal points of Christian activity in the early cathedral. The minaret of the mosque was also converted directly into a bell tower for the cathedral, with only cosmetic alterations such as the placement of a cross at its summit.

Notably, during the early period of the cathedral-mosque, the workers charged with maintaining the building (which had suffered from disrepair in previous years) were local Muslims (Mudéjars). Some of them were kept on payroll by the church but many of them worked as part of their fulfilment of a "labor tax" on Muslim craftsmen (later extended to Muslims of all professions) which required them to work two days a year on the cathedral building. This tax was imposed by the crown and was unique to the city of Cordoba. It was probably instituted not only to make use of Mudéjar expertise but also to make up for the cathedral chapter's relative poverty, especially vis-à-vis the monumental task of repairing and maintaining such a large building. At the time, Mudéjar craftsmen and carpenters were especially valued across the region and even held monopolies in some Castilian cities such as Burgos.

Other chapels were progressively created around the interior periphery of the building over the following centuries, many of them funerary chapels built through private patronage. The first precisely-dated chapel known to be built along the west wall is the Chapel of San Felipe and Santiago, in 1258. The Chapel of San Clemente was created in the southeast part of the mosque before 1262. A couple of early Christian features, such as an altar dedicated to San Blas (installed in 1252) and an altar of San Miguel (1255), disappeared in later centuries.

The first major addition to the building under Christian patrons is the Royal Chapel (Capilla Real), located directly behind the west wall of the Villaviciosa Chapel. It was begun at an uncertain date. While it is sometimes believed to have been started by Alfonso X, Heather Ecker has argued that documentary evidence proves it wasn't begun before the 14th century when Constance of Portugal, wife of Ferdinand IV, made an endowment for the chapel. It was finished in 1371 by Enrique II, who moved the remains of his father Alfonso XI and grandfather Ferdinand IV here. (Their remains were later moved in 1736 to the Church of San Hipólito.)

The Royal Chapel was constructed in a lavish Mudéjar style with a ribbed dome very similar to the neighbouring dome of the Villaviciosa Chapel and with surfaces covered in carved stucco decoration typical of Nasrid architecture at the time. This prominent use of the Moorish-Mudéjar style for a royal funerary chapel (along with other examples like the Mudéjar Alcázar of Seville) is interpreted by modern scholars as a desire by the Christian kings to appropriate the prestige of Moorish architecture in the Iberian Peninsula, just as the Mosque of Cordoba was itself a powerful symbol of the former Umayyad Caliphate's political and cultural power which the Castilians were eager to appropriate.

In the late 15th century a more significant modification was carried out to the Villaviciosa Chapel, where a new nave in Gothic style was created by clearing some of the mosque arches on the east side of the chapel and adding Gothic arches and vaulting. The nave is dated to 1489 and its construction was overseen by Bishop Íñigo Manrique. It originally had a series of Byzantine-Italian style frescoes by Alonso Martinez depicting saints and kings, but only one of these frescoes has been preserved to the present day and is now being kept at the Museum of Fine Arts in Cordoba.

The most significant alteration of all, however, was the building of a Renaissance cathedral nave and transept – forming a new Capilla Mayor [es] – in the middle of the expansive mosque structure, starting in 1523. The project, initiated by Bishop Alonso de Manrique, was vigorously opposed by the city council of Cordoba. The cathedral chapter eventually won its case by petitioning Charles V, king of Castile and Aragon, who gave his permission for the project to proceed. When Charles V later saw the result of the construction he is reputed to have been displeased, however, and is claimed to have commented: "You have built what you or anyone else might have built anywhere; to do so you have destroyed something that was unique in the world."

The architect Hernan Ruiz I was put in charge of the design of the new nave and transept. Before his death in 1547 he built the choir walls up to the windows and the gothic vaults on the south side. He also worked on the mosque building's eastern section (the extension added by Al-Mansur) by adding gothic vaulting to the mosque naves in this area. His son, Hernan Ruiz II "the Younger", took over the project after his death. He was responsible for building the transept walls to their full height as well as the buttresses upholding the structure. After him, the project was entrusted to architect Juan de Ochoa, who completed the project in a Mannerist style. The final element was the construction of the elliptical central dome of the transept, built between 1599 and 1607.

After the completion of Juan de Ochoa's work, Bishop Diego de Mardones initiated the construction of the main altarpiece and provided a significant donation himself for the project. The altarpiece was designed in a Mannerist style by Alonso Matías and construction began in 1618. Other artists who were involved in its execution included Sebastián Vidal, Pedro Freile de Guevara, and Antonio Palomino.

In 1589 a strong storm (or earthquake) caused damage to the former minaret, which was being used as a bell tower, and it was decided to remodel and reinforce the tower. A design by Hernán Ruiz III (son of Hernán Ruiz II) was chosen, encasing the original minaret structure into a new Renaissance-style bell tower. Some of the upper sections of the minaret were demolished in the process. Construction began in 1593 but eventually stalled due to resources being spent instead on the construction of the new cathedral nave and transept happening at the same time. Hernán Ruiz III died in 1606 and was unable to see its completion. The construction resumed under architect Juan Sequero de Matilla in 1616 and the tower was finished in 1617.

The new tower had imperfections, however, and required repairs only a few decades later in the mid-17th century. The cathedral hired architect Gaspar de la Peña to fix the problems. He reinforced the tower and modified the initial design of the Puerta del Perdón ("Door of Forgiveness") which passed through the tower's base. In 1664 Gaspar added a new cupola to the top of the belfry onto which he raised a statue of Saint Raphael made by the sculptors Pedro de la Paz and Bernabé Gómez del Río. In 1727 the tower was damaged by another storm and in 1755 pieces of it (mainly decorative details) were damaged by the 1755 Lisbon Earthquake. A French architect, Baltasar Dreveton, was charged with restoring and repairing the structure over a period of 8 years.

In March 1748 construction on the choir stalls of the Capilla Mayor began, with the commission awarded to Pedro Duque Cornejo. It was initially funded with the help of a large bequest by Archdeacon José Díaz de Recalde in 1742. Work on the choir stalls finished in 1757, though Duque Cornejo – who had worked on it continuously for nearly a decade – died just two weeks before the finished choir was officially opened.

In 1816 the original mihrab of the mosque was uncovered from behind the former altar of the old Chapel of San Pedro. Patricio Furriel was responsible for restoring the mihrab's Islamic mosaics, including the portions which had been lost. Further restoration works concentrating on the former mosque structure were carried out between 1879 and 1923 under the direction of Velázquez Bosco, who among other things dismantled the baroque elements that had been added to the Villaviciosa Chapel and uncovered the earlier structures there. During this period, in 1882, the cathedral and mosque structure was declared a National Monument. Further research work and archaeological excavations were carried out on the mosque structure and in the Courtyard of the Oranges by Félix Hernández between 1931 and 1936. More recent scholars have noted that modern restorations since the 19th century have partly focused on "re-islamicizing" (in architectural terms) parts of the Mosque-Cathedral. This took place in the context of wider conservation efforts in Spain, starting in the 19th century, towards studying and restoring Islamic-era structures.

The Mosque-Cathedral was declared a UNESCO World Heritage Site in 1984, and in 1994 this status was extended to the entire historic centre of Cordoba. A restoration project began on the bell tower in 1991 and finished in 2014, while the transept and choir of the Renaissance cathedral were also restored between 2006 and 2009. Further restorations of features like chapels and some of the outer gates have continued to take place up to the late 2010s.

The Great Mosque of Córdoba held a place of importance amongst the Islamic community of al-Andalus for centuries. In Córdoba, the Umayyad capital, the Mosque was seen as the heart and central focus of the city. To the people of al-Andalus "the beauty of the mosque was so dazzling that it defied any description."

After all of its historical expansions, the mosque-cathedral covers an area of 590 by 425 feet (180 m × 130 m). The building's original floor plan follows the overall form of some of the earliest mosques built from the very beginning of Islam. Some of its features had precedents in the Umayyad Mosque of Damascus, which was an important model built before it. It had a rectangular prayer hall with aisles arranged perpendicular to the qibla, the direction towards which Muslims pray. It has thick outer walls with a somewhat fortress-like appearance. To the north is a spacious courtyard (the former sahn), surrounded by an arcaded gallery, with gates on the north, west, and east sides, and fountains that replace the former mosque fountains used for ablutions. A bridge or elevated passage (the sabat) once existed on the west side of the mosque which connected the prayer hall directly with the Caliph's palace across the street. Al-Razi, an Arab writer, speaks of the valuable wine-coloured marble, obtained from the mountains of the district, which was much used in embellishing the naves of the mosque.

The Christian-era additions (after 1236) included many small chapels throughout the building and various relatively cosmetic changes. The most substantial and visible additions are the cruciform nave and transept of the Capilla Mayor (the main chapel where Mass is held today) which were begun in the 16th century and inserted into the middle of the former mosque's prayer hall, as well as the remodelling of the former minaret into a Renaissance-style bell tower.

The mosque-cathedral's hypostyle hall dates from the original mosque construction and originally served as its main prayer space for Muslims. The main hall of the mosque was used for a variety of purposes. It served as a central prayer hall for personal devotion, for the five daily Muslim prayers and the special Friday prayers accompanied by a sermon. It also would have served as a hall for teaching and for Sharia law cases during the rule of Abd al-Rahman I and his successors.

The hall was large and flat, with timber ceilings held up by rows of two-tiered arches resting on columns. The two-tiered arches consist of a lower tier of horseshoe arches and an upper tier of semi-circular arches.These rows of arches divided the original building into 11 aisles or "naves" running from north to south, later increased to 19 by Al-Mansur's expansion, while in turn forming perpendicular aisles running east–west between the columns. The nave that leads to the mihrab – which was originally the central nave of the mosque until Al-Mansur's lateral expansion of the building altered its symmetry – is slightly wider than the other naves, demonstrating a subtle hierarchy in the mosque's floor plan.

The original mosque had some 120 columns All of the original columns and capitals were reused from earlier Roman and Visigothic buildings, but subsequent expansions (starting with Abd al-Rahman II) saw the incorporation of new Moorish-made capitals that evolved from earlier Roman models. The current building has approximately 850 columns made of jasper, onyx, marble, granite and porphyry.

The mosque's architectural system of repeating two-tiered arches, with otherwise little surface decoration, is considered one of its most innovative characteristics and has been the subject of much commentary. The hypostyle hall has been variously described as resembling a "forest of columns" and having an effect similar to a "hall of mirrors". Scholar Jerrilynn Dodds has further summarized the visual effect of the hypostyle hall with the following:

Interest in the mosque's interior is created, then, not by the application of a skin of decoration to a separately conceived building but by the transformation of the morphemes of the architecture itself: the arches and voussoirs. Because we share the belief that architectural components must by definition behave logically, their conversion into agents of chaos fuels a basic subversion of our expectations concerning the nature of architecture. The tensions that grow from these subverted expectations create an intellectual dialogue between building and viewer that will characterize the evolving design of the Great Mosque of Cordoba for over two hundred years.

Much speculation has been offered on the inspiration for the arch design, often focusing on the possible influence of Umayyad architecture in the Levant, the homeland of Abd ar-Rahman I, where two-tiered arches were used in a simpler form. It is sometimes suggested that the design of the arches was meant to evoke a forest of palm trees from Abd ar-Rahman's youth in Syria. The motivation for the two-tiered design may have been more technical: unlike the large spolia columns available in Damascus, the columns available for reuse in Córdoba were not tall enough to raise the ceiling to the desired height on their own, so the addition of a second tier of arches above the first tier was an innovative way to resolve this. The precedent of multi-tiered arches was also present in the Iberian Peninsula thanks to remaining Roman aqueducts (e.g. the Milagros Aqueduct at Mérida); therefore, these local influences may have been more significant.

The voussoirs of the arches alternate between red brick and white stone. Colour alternations like this were also common in Umayyad architecture in the Levant and in pre-Islamic architecture on the Iberian Peninsula. According to Anwar G. Chejne, the arches were inspired by those in the Dome of the Rock. Horseshoe arches were known in the Iberian Peninsula in the Visigothic period (e.g. the 7th-century Church of San Juan de Baños) and to a lesser extent in Byzantine and Umayyad regions of the Middle East. The traditional "Moorish" arch in al-Andalus developed into its own distinctive and slightly more sophisticated version.