Nichiren Buddhism (日蓮系諸宗派: Nichiren-kei sho shūha) is a branch of Mahāyāna Buddhism based on the teachings of the 13th century Japanese monk Nichiren (1222–1282). Various forms of Nichiren Buddhism have had great influence among certain sections of Japanese society at different times in the country's history, such as among the merchants of Kyoto in Japan's Middle Ages and among some ultranationalists during the pre-World War II era. Nichiren Buddhism is generally noted for its focus on the Lotus Sutra and an attendant belief that all people have an innate Buddha nature and are therefore inherently capable of attaining enlightenment in their current form and present lifetime. It is also noted for positioning itself in opposition to other forms of Japanese Buddhism—in particular the Zen, Pure Land, esoteric, and Ritsu schools, which Nichiren saw as deviating from the orthodoxy of Mahayana Buddhism. Nichiren Buddhism is a comprehensive term covering several major schools and many sub-schools, as well as several of Japan's new religions. Nichiren Buddhists believe that the spread of Nichiren's teachings and their effect on practitioners' lives will eventually bring about a peaceful, just, and prosperous society.

Nichiren (日蓮) (February 16, 1222 – October 13, 1282) was a Buddhist monk who lived during the Kamakura period (1185–1333) in Japan. Nichiren taught devotion to the Lotus Sutra, entitled Myōhō-Renge-Kyō in Japanese, as the exclusive means to attain enlightenment and the chanting of Nam-Myōhō-Renge-Kyō as the essential practice of the teaching. Nichiren Buddhism includes various schools with diverging interpretations of Nichiren's teachings. While virtually all Nichiren Buddhist schools regard him as a reincarnation of the Lotus Sutra's Bodhisattva Superior Practices, Jōgyō Bosatsu (上行菩薩), some schools of Nichiren Buddhism's Nikkō lineages regard him as the actual Buddha of this age, or the Buddha of the Latter day of the Law.

Nichiren was born on February 16, 1222 in the village of Kominato, Nagase District, Awa Province (within present-day Chiba Prefecture). Nichiren's father was Mikuni-no-Tayu Shigetada, also known as Nukina Shigetada Jiro (d. 1258) and his mother was Umegiku-nyo (d. 1267). On his birth, his parents named him Zennichimaro (善日麿^?) which has variously been translated into English as "Splendid Sun" and "Virtuous Sun Boy" among others. The exact site of Nichiren's birth is believed to be submerged off the shore from present-day Kominato-zan Tanjō-ji (小湊山　誕生寺), a temple in Kominato that commemorates Nichiren's birth. In his own words about his birth, Nichiren stated that he was "the son of a chandala family who lived near the sea in Tojo in Awa Province, in the remote countryside of the eastern part of Japan."