was one of the honorary surnames bestowed by the Emperors of Japan of the Heian Period (794–1185 AD) on those of their sons and grandsons who were not considered eligible for the throne. The Taira clan were another such offshoot of the imperial dynasty. The Minamoto clan was also called the Genji Clan , using the alternate pronunciation of the Chinese Characters for Minamoto (gen) and uji, or family (ji).
The first emperor to start granting the name Minamoto was Emperor Saga. Afterwards Emperor Seiwa, Emperor Murakami, EmperorUda, and Emperor Daigo, among others, also gave their sons the name Minamoto. These specific hereditary lines coming from different emperors developed into specific clans referred to by the emperor's name followed by Genji, e.g. Seiwa Genji. According to some sources, the first to be given the name Minamoto was Minamoto no Makato, seventh son of Emperor Saga.
In 814 Emperor Saga (reigned 809–823) awarded the kabane Minamoto no Ason to his non-heir sons; thereafter, they and their descendants ceased to be members of the Imperial Family. Several subsequent emperors gave the Minamoto surname to their non-heir sons.
The most prominent of the several Minamoto families, the Seiwa Genji, descended from Minamoto no Tsunemoto (917–961), a grandson of the 56th Emperor Seiwa. Tsunemoto went to the provinces and became the founder of a major warrior dynasty. Minamoto no Mitsunaka (912–997) formed an alliance with the Fujiwara. Thereafter the Fujiwara frequently called upon the Minamoto to restore order in the capital, Heian-Kyo (or Kyoto.)
Mitsunaka's eldest son, Minamoto no Yorimitsu (948–1021), became the protégé of Fujiwara no Michinaga; another son, Minamoto no Yorinobu (968–1048) suppressed the rebellion of Taira no Tadatsune in 1032. Yorinobu's son, Minamoto no Yoriyoshi (998–1075), and grandson, Minamoto no Yoshiie (1039–1106), pacified most of northeastern Japan between 1051 and 1087.
The Seiwa Genji's fortunes declined in the Hōgen Rebellion (1156), when the Taira executed much of the line. During the Heiji Disturbance (1160), the head of the Seiwa Genji clan, Minamoto no Yoshitomo, died in battle. Taira no Kiyomori seized power in Kyoto by forging an alliance with the retired emperors Shirakawa and Toba and infiltrating the kuge. He sent Minamoto no Yoritomo (1147–1199), the third son of Minamoto no Yoshimoto of the Seiwa Genji, into exile. In 1180 Yoritomo mounted a full-scale rebellion against the Taira rule (Gempei or the Taira-Minamoto War), culminating in the destruction of the Taira and the subjugation of eastern Japan within five years. In 1192 he received the title shogun and set up the first bakufu at Kamakura.
Thus the Seiwa Genji line proved to be the most strong and dominant Minamoto line during the late Heian period with Minamoto no Yoritomo eventually forming the Kamakura Shogunate and becoming shogun in 1192. Also, it's from the Seiwa Genji line that the later Ashikaga (founders of the Ashikaga shogunate), Nitta, and Takeda clans come.
The protagonist of the classical Japanese novel The Tale of Genji, Hikaru no Genji, was bestowed the name Minamoto for political reasons by his father the emperor and was delegated to civilian life and a career as an imperial officer.