was a Japanese samurai, daimyo and the third shà Âgun of the Kamakura shogunate. He was the second son of the Kamakura shogunate founder, Minamoto no Yoritomo. His mother was Hà Âjà  Masako and his older brother was the second Kamakura shogun Minamoto no Yoriie.
His childhood name was . He was the last head of the Minamoto clan of Japan. His Dharma name was .
He was an accomplished waka poet.
After the death of Yoritomo in 1199, Sanetomo's maternal grandfather Hà Âjà  Tokimasa usurped the political and military power of the Shogunate, relegating the position and title of Sei-i Taishà Âgun, or shà Âgun, to a figurehead. Through hereditary succession, Sanetomo's older brother Yoriie became Sei-i Taishà Âgun in 1202, only to be stripped of the title a year later and put under house arrest for plotting against the Hà Âjà  clan. This was presumably to keep the shà Âgun a child and thus needing a regent (shikken) to make decisions in his place. Shortly thereafter, in 1203, Sanetomo became head of the Minamoto clan and was appointed Sei-i Taishà Âgun.
A year later, Yoriie was assassinated by the Hà Âjà Â. Sanetomo was never more than a puppet for his mother Hà Âjà  Masako and maternal uncle Hà Âjà  Yoshitoki, who used him as a pawn in their war with their father Tokimasa; Tokimasa tried to depose his grandson a number of times, beginning in 1205, causing Sanetomo to fear for his life thereafter.
Sanetomo, understanding his own powerlessness and not wanting to meet the same fate as his elder brother, put his time and energy into writing waka poetry and gaining posts within the powerless but honorary imperial court. Sanetomo was a talented poet, writing over 700 poems between the age 17 and 22 while he was tutored by Fujiwara no Teika. He published his private waka collection Kinkai Wakashà «, even having one of his tanka included in the anthology Ogura Hyakunin Isshu ("100 Poems by 100 Poets"), a collection of Japanese poems of the Heian and early Kamakura periods. Sanetomo reached the third-highest post of the imperial court, Udaijin (Minister of the Right or "vice-premier") in 1218.
Eventually, Sanetomo lapsed into inactivity and despair, plagued by fear of assassination and tormented by his chronic alcoholism (an addiction which Priest Eisai once tried to break by replacing alcohol with tea).
Under heavy snow on the evening of February 13, 1219 (Jà Âkyà « 1, 27th day of the 1st month), Sanetomo was coming down from the Senior Shrine at Tsurugaoka Hachiman-gà « after participating in a ceremony celebrating his nomination to Udaijin. His adopted heir and nephew, son of the deposed Yoriie, Kugyà Â, came out from beside the stone stairway of the shrine and assassinated him. For his act, he was himself beheaded a few hours later, thus bringing the Seiwa Genji line of the Minamoto clan and their rule in Kamakura to a sudden end.
Minamoto no Sanetomo was succeeded by Kujà  Yoritsune, son-in-law of Yoriie, as fourth shà Âgun of the Kamakura shogunate.
The years in which Sanetomo was shogun are more specifically identified by more than one era name or nengà Â.