The Gusuku Period (グスク時代, Gusuku jidai) is an era in the history of the Ryukyu Islands, an island chain now part of Japan. The period corresponds to the spread of agriculture and Japonic culture from Japan alongside increased social organization, eventually leading to endemic warfare and the construction of the namesake gusuku fortresses. Following the Shellmidden Period, the Gusuku is generally described as beginning in the 11th century after a dramatic social and economic shift over the previous centuries. The Shellmidden-Gusuku transition has been linked to Japonic-speaking migrants and influence from a Japanese trade outpost on the island of Kikai, leading to the emergence of the Proto-Ryukyuan language, the common ancestor of the modern Ryukyuan languages.

The period saw widespread agriculture across the archipelago, including the cultivation of foxtail millet, rice, barley, and wheat. Trade occurred with China, Korea, and Japan, including imports of ceramics and the export of mother of pearl. A unique form of vernacular architecture arose in the region, featuring elevated village houses, initially defended by palisades. The rise of the local nobility led to the steady expansion of fortifications, eventually evolving into the gusuku. The gusuku were large stone fortresses built in the hundreds across the archipelago, especially on Okinawa and Amami Ōshima.

By the 14th century, three kingdoms (the Sanzan) emerged on Okinawa as tributary kingdoms to the Ming dynasty of China. Rather than territorial states, these may have been confederations of powerful local rulers who declared themselves kings within the Chinese tribute system. In 1429, King Shō Hashi of Chūzan became the sole tribute king, although he likely failed to achieve complete political control over Okinawa. After a series of short-reigning kings and violent succession disputes, Shō Shin conquered much of the Ryukyu Islands and governed from Shuri Castle in Shuri, Okinawa, marking the start of the centralized Ryukyu Kingdom and the end of the Gusuku Period.

The Ryukyu Islands are an island chain on the eastern rim of the East China Sea, adjacent to the islands of Taiwan in the southwest and Kyushu to the northeast. The largest island of the chain is Okinawa, followed by Amami Ōshima (or simply Amami). During the glacial periods of the Pleistocene, land bridges connected the Ryukyus with the mainland via Taiwan, although the period these existed is disputed. The land bridges were gone by the time of the Late Pleistocene settlement of the Ryukyus around 32,000 years ago, requiring Paleolithic humans to have arrived at the islands via boats. Around 16,000 BCE, humans disappear from the archaeological record; they may have disappeared or been limited to the now-underwater coastline. Hunter-gatherer groups originating from Kyushu began to populate the northern and central Ryukyus around 5000 BCE, although recently-discovered sites suggest possible initial dates around 12,000 to 7000. This repopulation began the Shellmound or Shellmidden Period.

Complex hunter-gatherer societies emerged during the mid-Shellmidden, but polities such as chiefdoms did not form. This is attributed to the low population sizes of the islands before the spread of agriculture. The Shellmidden people exploited plentiful shellfish and reef fish populations, hunted Ryukyu wild boar, and possibly tended domestic pigs. Although other East Asian populations adopted agriculture long before the beginning of the Common Era, no cereal cultivation occurred in the Ryuykus before c. 800 CE, with plant foods largely limited to nuts. Cultivation of taro or other root crops has been theorized, although without conclusive archaeological evidence. The lone unambiguously farmed crops from the Shellmidden are bottle gourd seeds recovered from the Ireibaru site on Okinawa. Archaeological samples have found evidence for a limited amount of farming from the 800s onward.

While theories of more significant pre-Gusuku agriculture have been put forward, this would require the population to have later abandoned their dependence on agriculture before the beginning of the Gusuku period. Cultivation remained very limited until a rapid expansion in the tenth to twelfth centuries, corresponding to an influx of migrants from Japan.

Older sources use a later definition of the Gusuku Period, beginning c. 1200 CE and stretching well into the early Ryukyu Kingdom. However, following archaeological evidence for subsistence agriculture and greater social complexity prior to this date, 21st century sources have largely redefined the period as lasting from c. 1050 to c. 1429. This corresponds to the period of increased trade, societal shifts, and endemic warfare before the centralization of the kingdom of Chūzan and the unification of the Ryukyu Kingdom.

Within the Yaeyama Islands (or Sakishima more generally), the period corresponding to the Gusuku is sometimes labeled the Suku Period, divided between the Shinzato Mura Period (12th–13th centuries) and the Nakamori Period (13th–17th centuries). Due to their proximity and trade links to Kyushu, the Ōsumi Islands largely follow corresponding archaeological periods in Japan, adopting rice and millet cultivation during the middle of the Yayoi Period (c. 1st century CE).

Beginning around 300 BCE, the Shellmidden population saw a steady decline from its peak. Populations remained low across the 1st millennium CE. The islanders traded with Japan, but saw little cultural influence from it beyond pottery designs. During the 9th century, the Dazaifu (the regional Japanese government in Kyushu) established the Gusuku site on the island of Kikai as a trading outpost. Exploiting the lucrative trade of turbo sea snail shells (a source of mother of pearl highly prized by artisans), Kikai became a major trading hub connected to the Japanese port of Hakata and the Korean kingdom of Goryeo. A small community of merchants from Goryeo settled on Kikai. Their settlement led to the creation of Kamuiyaki ware, a new form of stoneware with cultural influence from Korean ceramics. The Gusuku site became the center of a small state encompassing Kikai and portions of neighboring Amami.

Beginning in the 11th century, large numbers of agrarian Japonic-speaking peoples settled the Ryukyus, with Kikai as the origin of various successive migration waves across the archipelago. These migrations, while all originating on Kikai, spread progressively southward. The Amami Islands were the first to be fully settled, followed by the Okinawa Islands, the Miyako Islands, and finally the Yaeyama Islands. This migration was likely motivated by access to various trade goods found in the southern islands, highly coveted in Song China and by the aristocracy of Heian Japan.

The indigenous population of the Ryukyu Islands prior to the Gusuku Period was of the non-Japonic Jōmon ancestry, with little of the Yayoi genetics prevalent in mainland Japan. Interactions between this population and the Japonic newcomers varied across the archipelago. However, many Japonic communities formed cultural enclaves, evidenced by their material cultures coexisting for several centuries. In some cases, Japonic settlements were founded with practically no indigenous influence. Indigenous culture on the islands gradually assimilated, and vanished entirely by the 14th century.

Due to their shared set of innovations absent in Japanese, the modern Ryukyuan languages are generally thought to form one of the two or three main branches of Japonic, descended from a common Proto-Ryukyuan origin. They retain archaic features from Proto-Japonic that were lost in Old Japanese, suggesting a divergence date no later than the 7th century. However, Sino-Japanese vocabulary borrowed from Early Middle Japanese indicates that it maintained close contact with Japanese until the 8th or 9th century. This linguistic divergence long before the Gusuku Period suggests a pre-Proto-Ryukyuan homeland in southern Kyushu and the surrounding islands. Proto-Ryukyuan itself branched off from this earlier form within the Ryukyu Islands, possibly beginning on Kikai, and diversified as it spread across the archipelago.

Earlier, now-discredited, theories attribute the emergence of Proto-Ryukyuan to either the Japonic-speaking Hayato people settling the Ryukyus after the conquest of southern Kyushu by the Yamato state, or as an evolution from a trade pidgin on Kikai.

As the Shellmidden Period transitioned into the Gusuku, subsistence agriculture emerged. Agriculture likely took root in the Amami Islands in the 8th century, before spreading to the Okinawa Islands 100–200 years later. Rice and millet agriculture spread to Sakishima by the 12th century.

Cereal crops such as rice, barley, wheat, and foxtail millet have been found in Gusuku Period sites, alongside possibly beans. Sites in Southern Okinawa mainly grew millet and barley, while rice predominated in northern Okinawa and Amami. This rice was initially japonica rice, but tropical varieties were likely introduced later via trade with China and Southeast Asia. Initially in low-lying alluvial regions, farms gradually shifted to higher slopes. Wheat and barley were mainly grown through dryfield cultivation, with irrigation limited to the rice paddies. Cattle were used to cultivate both varieties of field. Archaeological examinations of Gusuku Period sites on Miyako Island have revealed similar crops to Okinawa and Amami. Foxtail millet composes the vast majority of finds at Miyako, alongside smaller amounts of Adzuki beans and broomcorn millet.

The centrality of agriculture to Gusuku Period society is a topic of academic contention. Historians and archaeologists have generally analyzed the Gusuku as a stratified agrarian society, attributing the formation of states and nobility to this agricultural base. Others have disputed this, suggesting that local agriculture was unlikely to produce a significant surplus, and instead attributing these developments to maritime trade.

Shellmidden-era construction was largely limited to pit-houses. The larger settlements of the Gusuku saw the flourishing of vernacular architecture. Settlements during the 11th to 13th centuries typically comprised several elevated main houses raised on posts with diameters of 50 centimeters (1.6 ft) or more. Pillars within the house were typically spaced by a bay (ken) of roughly 1 metre (3.3 ft), significantly smaller than the ken spacings used in traditional Japanese architecture. Houses contained hearths, with the largest having two. Elevated storehouses (called takakura) were located 10–15 meters (30–50 ft) from the main structures, generally to the southwest to maximize sunlight. Some of these village sites include the remains of metalworking facilities, including pits for the storage of ironsand and hearths equipped with clay tuyeres.