The Genpei War (源平合戦, Genpei Kassen, Genpei-Gassen; 1180–1185) was a national civil war between the Taira and Minamoto clans during the late Heian period of Japan. It resulted in the downfall of the Taira and the establishment of the Kamakura shogunate under Minamoto no Yoritomo, who appointed himself as Shōgun in 1192, governing Japan as a military dictator from the eastern city of Kamakura.

It followed a coup d'état by the Taira in 1179 with the removal of rivals from all government posts, and subsequently banishing them, and a call to arms against the Taira, led by the Minamoto in 1180. The ensuing Battle of Uji took place just outside Kyoto, starting a five-year-long war, concluding with a decisive Minamoto victory in the naval Battle of Dan-no-ura. However, it has been pointed out that the Battle of Ōshū in 1189 was the last battle during this period of civil war, as it completed Yoritomo's nationwide domination through the annexation of Northeast Japan.

The name "Genpei" (Japanese pronunciation: [ɡem.pei, -peː], sometimes romanized as Gempei) comes from alternate readings of the kanji "Minamoto" (源 Gen) and "Taira" (平 Hei, pronounced as the second element in some compounds as -pei). The conflict is also known in Japanese as the Jishō–Juei War (治承・寿永の乱, Jishō–Juei no Ran), after the two Imperial eras between which it took place. The term Genpei Kassen is sometimes used in Japan, but it has been argued that it is not appropriate to use the term "Genpei" for this war.

The Genpei War was the culmination of a decades-long conflict between the two aforementioned clans over dominance of the Imperial court and, by extension, control of Japan. In the Hōgen Rebellion and in the Heiji Rebellion of earlier decades, the Minamoto attempted to regain control from the Taira and failed.

In 1180, Taira no Kiyomori put his grandson Antoku (then only 2 years of age) on the throne after the abdication of Emperor Takakura. Emperor Go-Shirakawa's son Mochihito felt that he was being denied his rightful place on the throne and, with the help of Minamoto no Yorimasa, sent out a call to arms to the Minamoto clan and Buddhist monasteries in May. However, this plot ended with the deaths of Yorimasa and Mochihito.

In June 1180, Kiyomori moved the seat of imperial power to Fukuhara-kyō, "his immediate objective seems to have been to get the royal family under his close charge."

The actions of Taira no Kiyomori having deepened Minamoto hatred for the Taira clan, a call for arms was sent up by Minamoto no Yorimasa and Prince Mochihito. Not knowing who was behind this rally, Kiyomori called for the arrest of Mochihito, who sought protection at the temple of Mii-dera. The Mii-dera monks were unable to ensure him sufficient protection, so he was forced to move along. He was then chased by Taira forces to the Byōdō-in, just outside Kyoto. The war began thus, with a dramatic encounter on and around the bridge over the River Uji. This battle ended in Yorimasa's ritual suicide inside the Byōdō-in and Mochihito's capture and execution shortly afterwards.

It was at this point that Minamoto no Yoritomo took over leadership of the Minamoto clan and began traveling the country seeking to rendezvous with allies. Leaving Izu Province and heading for the Hakone Pass, he was defeated by the Taira in the battle of Ishibashiyama. However, he successfully made it to the provinces of Kai and Kōzuke, where the Takeda and other friendly families helped repel the Taira army. Meanwhile, Kiyomori, seeking vengeance against the Mii-dera monks and others, besieged Nara and burnt much of the city to the ground.