Sōhei (僧兵; "monk soldiers", "warrior monks") were Buddhist warrior monks of both classical and feudal Japan. At certain points in history, they held considerable power, obliging the imperial and military governments to collaborate.

The prominence of the sōhei rose in parallel with the ascendancy of the Tendai school's influence between the 10th and 17th centuries. The warriors protected land and intimidated rival schools of Buddhism, becoming a significant factor in the spread of Buddhism and the development of different schools during the Kamakura period.

The sōhei shared many similarities with the European lay brothers, members of a monastic order who might not have been ordained. Much like the Teutonic Order, the warrior monks of the Holy Roman Empire, and the crusading orders, sōhei did not operate as individuals, or even as members of small, individual temples, but rather as warriors in a large extended brotherhood or monastic order. The home temple of a sōhei monastic order might have had several, if not dozens or a hundred, smaller monasteries, training halls, and subordinate temples connected to it. A famous sōhei monastery is Enryaku-ji on Mount Hiei, just outside Kyoto, while Kōfuku-ji in Nara also fielded a large sōhei army.

Warrior monks first appeared during the Heian period, when bitter political feuds began between different temples, different subsects of Buddhism, over imperial appointments to the top temple positions in the sōkan system (zasu or Buddhist abbot). Much of the fighting over the next four centuries was over these sorts of political feuds, and centered around the temples of Kyoto, Nara, and Ōmi, namely the Tōdai-ji, Kōfuku-ji, Enryaku-ji, and Mii-dera, the four largest temples in the country.

The first armed conflict broke out in 949, when 56 monks from Tōdai-ji staged a protest at the residence of a Kyoto official, over an appointment that displeased them. Protests of this sort continued through the 10th century, often breaking out into brawls in which some participants would be killed. In 970, following a dispute between Enryaku-ji and the Yasaka Shrine of Kyoto, the former established the first standing army of warrior monks. It is not entirely clear whether this standing army consisted of monks from Enryaku-ji or was more like a mercenary army, since Ryōgen, the abbot who established this army, also established a code of monastic conduct that prevented monks from leaving Mount Hiei during their twelve-year training, from covering their faces, and from carrying weapons.

Beginning in 981, there were a number of armed conflicts between Enryaku-ji and Mii-dera, each the head temple of a different sub-sect of Tendai. These disputes were, as before, over political appointments and dishonorable etiquette. More often than not, these were cases of members of one faction being chosen as the abbot of the other faction's temple, and the monks would protest. This continued, on and off, once stopping for as long as 40 years, through the 11th and into the 12th century. The armies became larger and the violence increased, until in 1121 and 1141 Mii-dera was burned to the ground by monks from Enryaku-ji. Other temples became embroiled in the conflicts as well, and Enryaku-ji and Mii-dera united against Kōfuku-ji, and, another time, against Kiyomizu-dera.

At the end of the 12th century, Japan was plunged into the Genpei War and, while the feuds between the temples did not end, they became subsumed by larger events. The warring Minamoto and Taira clans both tried to obtain the aid of the warrior monks of Nara and Kyoto, adding the temples' forces to the clans' already mighty armies of samurai.

Taira no Kiyomori sent generous gifts of rice and silk to Enryaku-ji, ensuring they would not help his enemies, the Minamoto, who had allied themselves with the monks of Mii-dera. In the Battle of Uji in 1180, one of the more famous battles in which sōhei participated, the monks of Mii-dera, along with a force of Minamoto samurai, tried to defend the bridge over the Uji River, and the Byōdō-in, a temple behind it, from an attacking Taira force. The monks pulled up the planks of the bridge to impair the ability of the horse mounted samurai to cross. The warrior monks stood their ground with bow and arrow, naginata, sword and dagger, but were ultimately defeated. Following his victory, Taira no Kiyomori ordered that revenge be taken upon the monks that opposed him. Mii-dera was burned to the ground once again, as were many of the temples of Nara. Only the Enryaku-ji escaped unscathed.