The Mito Rebellion (水戸幕末争乱 Mito bakumatsu sōran), also known as the Kantō Insurrection or the Tengutō Rebellion (天狗党の乱 Tengutō no Ran), was an uprising by the pro-imperial, anti-shogunate Tengutō faction in the Mito Domain (present-day Ibaraki Prefecture) in 1864. The culmination of a long and bitter factional rivalry within the domain, it resulted in more than 1,300 casualties in Mito and hundreds of deaths among troops from other domains. It devastated Mito economically, militarily and politically. Along with the Chōshū Expeditions of 1864 and 1866, the Boshin War of 1868/69, it was one of the three significant confrontations leading to the disintegration of the Tokugawa shogunate.

Throughout the Edo Period (1600-1868) factional rivalries and outbursts of unrest had been quite common in many domains. In Mito, the rivalries were triggered by ideological tensions and other political issues. The ideological element stemmed from the scholarly tradition of the Dainihonshi (大日本史, lit. "Great History of Japan"), the monumental history of Japan undertaken by Tokugawa Mitsukuni, and the pro-imperial tendencies fostered by the Mito School of learning.

During the nineteenth century, as foreign vessels intruded Japanese waters more and more frequently, Mito scholars such as Aizawa Seishisai (会沢 正志斎, 1781-1863) advocated fundamental reforms to enable Japan to cope with both growing domestic problems and increasing pressure from foreign nations to open the country up. In 1829, Tokugawa Nariaki became daimyō of Mito, launching a vigorous programme to achieve those aims. Nariaki's ambitious policies created intense political conflict in the domain, resulting in polarisation between a conservative faction impeding many of his plans and a radical faction (天狗党 Tengutō) bitterly resenting their resistance.

When Mito's internal disputes interfered in the shogunate's Tenpō reforms (天保の改革 tenpō no kaikaku), Nariaki was forced to resign in 1844, only to return to active politics during the diplomatic crisis ensuing the arrival of Commodore Perry and his black ships in 1853. Abe Masahiro (阿部正弘, 1819-1857), the senior chief councillor of the shogunate, solicited Nariaki's cooperation, but his influence on national affairs, in particular, his support of the imperial camp, led to renewed tensions in Mito. When Ii Naosuke (井伊直弼, 1815-1860), the shogunal regent, violently repressed all pro-imperial agitation in what has become known as the Ansei Purge (安政の大獄 Ansei no taigoku), tensions all over the country, also in Mito, boiled over. Ii's assassination by samurai from Satsuma and Mito in 1860 calmed emotions only temporarily.

When the shogunate in 1863 failed to comply with imperial court orders to expel all foreigners, Mito's Tengutō faction strongly criticised the government as well as the more cautious factions in Mito Domain and advocated forceful expulsion. Months of political manoeuvring finally prompted open confrontations: Fujita Tōko's son, Fujita Koshirō (藤田小四郎, 1842-1865) and others led Tengutō forces southwest to Mount Tsukuba, defying orders of their daimyō Tokugawa Yoshiatsu and the shogunate to return home, convincing the shogunate and the Mito conservatives under Ichikawa Sanzaemon (市川 三左衛門, 1816–1869) to take action. By May 1864 Fujita's band had grown to hundreds of supporters under the pro-imperial supporter Takeda Kōunsai (武田耕雲斎, 1803-1865), who at first tried to calm the insurgents, but later agreed to lead them on a march to Kyōto to request the shogun to follow the principles of son no jōi (尊皇攘夷, "Revere the Emperor, Expel the Barbarians").

The shogunate mobilised all available forces to quell what had begun as an intradomain quarrel but now had turned into a conflict of proportions of a major insurrection against the Tokugawa regime. The shogunate ordered all neighbouring daimyō forces in the Kantō region to support them militarily or at least give moral support. Only in autumn 1864, the other domains obeyed and mobilised their troops against the insurgents. After a series of defeats, the shogunal forces gained the upper hand and finally forced the pro-imperial insurgents to surrender in December 1864. Twelve thousand troops, including Ichikawa's Mito forces and units from around six other domains, were needed to crush some two thousand insurgents.

The insurgents were punished ruthlessly, Fujita Koshirō, Takeda Kōunsai and 353 others were beheaded, and for the next three years, the conservative faction ruled Mito with a heavy hand. The surviving members of the Tengutō faction resurfaced during the Meiji Restoration of 1868, taking bitter revenge on their former oppressors. The internal conflicts of the Mito Domain lasted for decades to come.