The Omar Ali Saifuddien Mosque (Malay: Masjid Omar Ali Saifuddien) or unofficially Sultan Omar Ali Saifuddin Mosque (SOAS Mosque), is a mosque in Bandar Seri Begawan, the capital of Brunei. Omar Ali Saifuddien III, the 28th Sultan of Brunei, is credited with building the mosque, thus its name. It is one of the two state mosques (masjid negara), the other Jame' Asr Hassanil Bolkiah Mosque. One of the biggest and most striking mosques in the Far East, it was completed in 1958 at a cost of $5 million USD. The Mughal architecture of India had a significant effect on the design.

The mosque is recognisable by its gold dome that dominates the city's skyline. It is situated in the middle of a man-made lagoon with a ceremonial stone barge. The inside is decorated with rugs from Saudi Arabia and walls made of Italian marble. The mosque is a popular tourist destination due to its façade, but its main purpose is to be a place of worship. It has since become the most photographed icon in the country.

According to stories, Rudolfo Nolli, an Italian artist and architect, created the mosque based on an initial drawing created by Sultan Omar Ali Saifuddien III. With the help of a contractor, Sino-Malayan Engineer, and comprehensive architectural designs created by the Malaysian-based Booty and Edward Chartered Architects, the construction began on 4 February 1954. The consulting firm was the Singapore-based Steen, Sehested and Partners.

Sultan Omar Ali Saifuddien III provided the first concepts for the mosque designs, which draughtsman Haji Besar bin Sagap put down on paper. Since Padang Besar (present day Taman Haji Sir Muda Omar 'Ali Saifuddien) was utilised for large meetings, it was first proposed that this was the location of the mosque. For the sake of the Kampong Ayer people, the Sultan, however, requested that it be in the current location close to the Brunei River. It so happens that Sultan announced the 1st National Development Plan (RKN 1) when he built this mosque.

The construction uses 1,500 tons of concrete and 700 tons of steel, with its foundation piles measuring between 80–120 feet (24–37 m). The mosque was completed in five years, and the Sultan declared it open on 26 September 1958, in conjunction to his 42nd birthday celebration. Hassan Azhari sang the call to prayer at the mosque's opening. The opening ceremony was attended by Malay Sultans, state dignitaries, the Prime Minister and Deputy Prime Minister of Malaysia, the Governor General of Southeast Asia, and representatives from the British government, and it was notably the first news reported over the radio in the country at that time.

Between 30 December 1959 and 3 January 1960, Anwar Musaddad gave several lectures in the mosque. The mosque's barge was inaugurated on 18 December 1967 in conjunction with the 1,400th anniversary of Nuzul Al-Quran. Prominent people include Queen Elizabeth II in 1972, Pierre Trudeau in 1983, Muhammad Zia-ul-Haq in the mid-1980s, Prince Salman bin Abdul Aziz in 1998, and Narendra Modi in 2024, have all paid the mosque a visit.

During the construction of the Jame' Asr Hassanil Bolkiah Mosque in 1994, the mosque underwent its first renovation. It was carried out in January 1994, and involved the installation of air conditioning and decorative glass with verses from the Al-Quran on the small windows of the domes, some of which displayed the Asma' Allah al-Husna. The mosque's exterior hasn't changed in 50 years because the previous glassless windows allowed fresh air to circulate inside.

The mosque has hosted a number of important occasions in Brunei's history, such as the historic 31 December 1983, mass thanksgiving prayer on the declaration of Independence Day. It remains the main venue for the solemnisation of royal weddings and mass prayers of gratitude on National Day. Sultan Hassanal Bolkiah on 28 July 1965; Crown Prince Al-Muhtadee Billah on 9 September 2004; Princess Rashidah Sa'adatul Bolkiah on 15 August 1996; and Princess Majeedah Nuurul Bolkiah on 7 June 2007 are among the notable royal marriages that took place in the mosque.

On 26 September 2008, the mosque celebrated its golden jubilee.

The mosque's general architectural style is primarily influenced by the Mughal Empire, with elements of Islam and Brunei accented throughout the structure's façade and interior. The mihrab, mimbar, prayer hall and dome, minaret, ablution area, and courtyard are the six areas that make up the mosque. Each portion was purposefully designed for a certain function. Nolli created all of the architectural elements, including the pillars with their distinctive Bruneian motif, a thick rope or kalat. This pattern is a reflection of the columns in Lapau. A square fountain pool with green and yellow tiles creating a songket weave (tenunan) pattern can be seen outside in the courtyard on the left.

The mosque, which measures 225 by 86 feet (69 m × 26 m) and can hold 3,000 worshipers, features an area for ablution that is surrounded by kalat pillars that are evocative of historic Andalusian palaces. Islamic art, including Arabic calligraphy, geometric patterns, floral motifs, trefoil arches, and Quranic passages, adorn the mosque's interior as visitors enter through its copper doors. The mihrab marks the direction of Mecca (qibla) and is located at the far end of the prayer hall. It is here that the imam leads the assembly in prayer. The mosque's central architectural and symbolic element, the mihrab, is adorned with gold mosaics and abstract floral designs. Its ceiling is a circular design with Quranic passages.

The mimbar, which the Sultan created, is a two-story pulpit in the mosque where the Imam gives sermons. The al-Muazzin, who invites people to prayer, uses the first step while the imam uses the second level. Together with the four lesser minarets, the 166 foot (51 m)-tall main minaret, the highest in the nation, represents the five pillars of Islam and amplifies the call to prayer. The prayer hall is located beneath the 52 metres (171 ft)-tall golden dome and is decorated with handcrafted Axminster carpets from Saudi Arabia and Belgium. There is also a second royal prayer chamber for royal family members on the top right level that may be reached via escalator.

The mosque's other furnishings consisted of the S$200,000 Italian marble covering its floors and columns, Shanghai granite, English chandeliers, and stained glass, are combined. The dome's curving surface covered in a mosaic made of more than three million Venetian glass pieces is perched above the cream-coloured building. Anthony Burgess said in his memoirs that the dome was coated in gold leaf, which came away in flakes as the building contracted and expanded, giving the local fishermen the impression that the gold was a gift from Allah. An elevator up the 44 metres (144 ft)-tall minaret was also built. The chandelier of 15 feet (4.6 m) in diameter and weighing more than 3 tonnes (3.0 long tons; 3.3 short tons); it holds 62 fluorescent tubes, with an addition of 480 tubes for the interior.

Situated next to the Kedayan River and surrounded by a man-made lagoon, the mosque represents a bridge connecting the country's past and present. The most famous example is the concrete re-creation of Sultan Bolkiah's boat, a 16th century Mahligai (royal barge) on the lagoon. This B$500,000 barge, which has been accessible since 1967, is based on the traditional longboats used for warfare, memorial services, and royal celebrations by the Borneo's indigenous people. Its central hypostyle pavilion, stern, and bow are designed like birds, and its roof is pyramidal. These jars are commonly found in European drawings, woodcarvings, cloth paintings, and murals. The lagoon around the mosque and its boat path are ornamented with ridges that mimic the ancient shield known as the kelasak. Bruneian textile weaving patterns and royal regalia serve as the inspiration for the boat's elaborate floral and vegetal decorations. The main pavilion's finial features Sultan Sharif Ali's royal emblem. Mosaics that imitate the vibrant hues, intricate patterns, and motifs of Brunei's songket textiles, which are highly valued and essential to royal customs, are also used to decorate the mosque's water fountain and ablution area.

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  The mosque seen from Taman Mahkota Jubli Emas
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  The Department of Mosque Affairs building alongside the mosque
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  The mosque and mahligai in the foreground
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  The mosque's prayer hall
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  The mosque at dusk
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  Close-up of the minarets