wife of Minamoto Yoritomo (1147–99), the first shogun, or military dictator, of Japan. She is said to have been largely responsible for Yoritomo's success, and after his death she assumed great power.

Masako fell in love with Yoritomo when he was exiled from the capital by his family's rivals, the Taira clan, and put under the watch of Masako's father. She persuaded her father to allow her to marry him, and later she aided Yoritomo in his conquest of Japan.

After Yoritomo's death (1199), Masako became a nun. In 1204 her oldest son, Yoritomo's successor Yoriie, was assassinated. His brother Sanetomo, Masako's second son, succeeded him as shogun; real power, however, was exercised through the post of regent, which was held by Masako's father, Tokimasa.

The next year, an attempt was made on Sanetomo's life, but Masako saved him. Tokimasa, who was implicated in the attempt, was deposed, and Masako's brother Yoshitoki became the new regent, with Masako exercising considerable indirect power. When Sanetomo was assassinated in 1219 by one of Yoriie's sons, Masako and her brother had an infant great-grandson of Yoritomo set up as a puppet shogun. She continued to dominate the government until her death, her power being so great that she was called the ama (“nun”) shogun.