Hongan-ji (本願寺 Temple of the Primal Vow?), also archaically romanized as Hongwanji, is the collective name of the largest school of Jōdo Shinshū Buddhism (which further sub-divides into the Nishi and Higashi branches). 'Hongan-ji' may also refer to any one of several actual temple buildings associated with the sect.
The Hongan-ji was established as a temple in 1321, on the site of the Otani Mausoleum, where Shinran, the founder of the Jōdo Shinshū ("True Pure Land") was buried. The mausoleum was attended by Shinran's grandson (through daughter Kakushinni), Kakue. Kakue's own son, Kakunyo, became the first chief priest of the Hongan-ji and third monshu (spiritual leader), and dedicated it to the worship of Amitābha (Amida). The Hongan-ji first gained power and importance in the 15th century, when Rennyo became its eighth monshu. However, the Tendai based on Mount Hiei saw this expansion as a threat and attacked the Hongan-ji three times with their army of sōhei. Rennyo fled to Yoshizaki-gobō, where he established a new temple compound.
During the Sengoku period, fearing the power of the monks of the Hongan-ji, Oda Nobunaga tried to destroy it. For ten years, he laid siege to the Ishiyama Hongan-ji in Osaka, one of the two primary temple fortresses of the sect.
In 1602, just after Tokugawa Ieyasu became shogun, he declared that Hongan-ji be split in two. Kyonyo, the twelfth monshu of Hongan-ji, became the first head of the new Higashi Honganji (東本願寺 "Eastern Honganji"), while his younger brother Junnyo became the twelfth chief priest of the Nishi Honganji (西本願寺, "Western Honganji"; sometimes called the Hompa Honganji 本派本願寺).