The Ōnin War (応仁の乱, Ōnin no Ran), also known as the Upheaval of Ōnin and Ōnin-Bunmei war, was a civil war that lasted from 1467 to 1477, during the Muromachi period in Japan. Ōnin refers to the Japanese era during which the war started; the war ended during the Bunmei era. A dispute between a high official, Hosokawa Katsumoto, and a regional lord, Yamana Sōzen, escalated into a nationwide civil war involving the Ashikaga shogunate and a number of daimyō (大名, feudal lords) in many regions of Japan.

The war initiated the Sengoku period, "the Warring States period." This period was a long, drawn-out struggle for domination by individual daimyō, resulting in a mass power-struggle between the various houses to dominate the whole of Japan.

The Ōnin conflict began as a controversy over who would succeed shōgun Ashikaga Yoshimasa. In 1464, Yoshimasa had no heir. He persuaded his younger brother, Ashikaga Yoshimi, to abandon the life of a monk, and named him heir. In 1465, the unanticipated birth of a son to Yoshimasa put these plans in question. The infant, Yoshihisa, led to a succession crisis with two competing factions. On one side was the shōgun and his brother, together with the shōgun's deputy, Hosokawa. On the other side was Yoshihisa's mother, Hino Tomiko, and her ally Yamana, who was the governor of several provinces.

Tomiko sought political and military support to rule as regent until the maturity of her son, the future shogun Ashikaga Yoshihisa. She secured the support of Yamana Sōzen and other leaders of powerful samurai clans. In contrast to Tomiko and Yamana, Yoshimi had the support of the Hosokawa clan, a powerful clan that had a great influence on the shogunate court.

The cause of the war is often attributed to the struggle to succeed the 8th shogun, Ashikaga Yoshimasa, but in fact there were multiple causes. The real power of the Ashikaga shogunate was vested in a council of powerful daimyo, but the deaths of a number of influential daimyo and the intervention in politics of women close to the shogun's entourage led to chaos in the shogunate, and from 1441 on, the masses demanded a virtue decree every few years and destroyed the sake stores and warehouses that were the source of the shogunate's funding. Under these circumstances, in addition to the conflicts over the succession of the shogun, conflicts over the succession of the Hatakeyama clan and conflicts over the succession of the Shiba clan occurred simultaneously, all of which contributed to the war. According to the most popular theory, the main cause of the war was the struggle for succession between Hatakeyama Yoshinari and Hatakeyama Masanaga within the Hatakeyama clan, with the participation of Hosokawa Katsumoto, Yamana Sōzen and other daimyo from various regions.

In 1467, these conflicts finally led to the Ōnin War between the Eastern Army, led by Hosokawa Katsumoto and including Hatakeyama Masanaga, Shiba Yoshitoshi, and Ashikaga Yoshimi, and the Western Army, led by Yamana Sōzen and including Hatakeyama Yoshinari, Shiba Yoshikado, and Ashikaga Yoshihisa.

At first, the Eastern Army supported Ashikaga Yoshimi and the Western Army supported Ashikaga Yoshihisa as the next 9th shogun. However, Ashikaga Yoshimi, who disliked war, fled to Ise to seek refuge with the Kitabatake clan, and Ashikaga Yoshimi fell out with Ashikaga Yoshimasa, the 8th shogun. When the Western Army received Ashikaga Yoshimi in 1468, the Eastern Army came to support Ashikaga Yoshihisa. In other words, the successors supported by the Eastern and Western armies reversed within a year.

"Just what, we must often ask, did the contenders want? I don't think they knew. Certainly, they understood the particular enmities and prizes that moved them. Surely, too, they perceived links between their immediate purposes and the deeper strains in every relationship of power and personal attachment. All else remained obscure."