The Sengoku period (戦国時代, Sengoku jidai; Japanese pronunciation: [seŋ.ɡo.kɯ (d)ʑiꜜ.dai, -ŋo.kɯ-] lit. 'Warring States period') was the period in Japanese history in which civil wars and social upheavals took place almost continuously in the 15th and 16th centuries. The Kyōtoku incident (1454), Ōnin War (1467), or Meiō incident [ja] (1493) are generally chosen as the period's start date, but there are many competing historiographies for its end date, ranging from 1568, the date of Oda Nobunaga's march on Kyoto, to the suppression of the Shimabara Rebellion in 1638, deep into what was traditionally considered the Edo period. Regardless of the dates chosen, the Sengoku period overlaps substantially with the Muromachi period (1336–1573).

This period was characterized by the decline of the Ashikaga shogunate, the de facto central government, while the sengoku daimyo (戦国大名, feudal lords of the Sengoku period), local power-holders, acquired greater political influence. The people rebelled against the feudal lords in revolts known as Ikkō-ikki (一向一揆, Ikkō-shū uprising).

The period saw a breakdown in the traditional master-servant relationship between a lord and his vassals, with many instances of vassals rebelling against their lords, internal dynastic conflict over lordships within noble families (in which vassals would take sides), and the installation of figurehead lords by cadet branches of noble families. The period was also marked by the loosening of samurai culture, with people born into other social strata sometimes making a name for themselves as warriors and thus becoming samurai. In turn, events sometimes allowed common samurai to rise to the rank of sengoku daimyo; these included Hōjō Sōun (the first to do so), and Uesugi Kenshin, a Shugodai (守護代, deputy Shugo) who attained power by weakening and eventually replacing his lord. The most spectacular example of a sengoku-era rise is often considered to be that of Toyotomi Hideyoshi, who rose from a peasant background to successively become a samurai, sengoku daimyo, and kampaku (Imperial Regent).

Modern Japan recognizes Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu as the three "Great Unifiers" (三英傑／さんえいけつ) for their restoration of Japan's central government.

During this period, although the Emperor of Japan was officially the ruler of the state and every lord swore loyalty to him, he was largely a marginalized, ceremonial, and religious figure who delegated power to the shōgun, a noble who was roughly equivalent to a military dictator.

From 1346 to 1358, during the Nanboku-cho period, the Ashikaga shogunate gradually expanded the authority of the Shugo (守護), the local military and police officials established by the Kamakura shogunate, giving the Shugo jurisdiction over land disputes between gokenin (御家人) and allowing the Shugo to receive half of all taxes from the areas they controlled. The Shugo shared their newfound wealth with the local samurai, creating a hierarchical relationship between the Shugo and the samurai, and the first early daimyo (大名, feudal lords), called shugo daimyo (守護大名), appeared.

In 1428, Ashikaga Yoshimochi, the fourth Ashikaga shogun, was ill and the question of his succession arose. Ashikaga Yoshikazu, the 5th Ashikaga shogun, died of illness at the age of 19, so the 6th Ashikaga shogun was chosen from among Yoshimochi's four brothers, and to ensure fairness, a lottery was held. The sixth Ashikaga shogun was Ashikaga Yoshinori. But he was not educated to be a shogun, and his temperamental and despotic behavior caused resentment. Akamatsu Mitsusuke assassinated him during the Kakitsu Rebellion. This led to instability in the Ashikaga shogunate system. The shogunate gradually lost influence and control over the daimyo.

The beginning of the Sengoku Period is considered to be the Kyōtoku incident, Ōnin War, or Meiō incident.