Sangawa Motomasa served as a bushō during the Sengoku period. He was the lord of Hirune Castle in Sanuki Province.
Motomasa was born as the son of Sangawa Motoie. Around the time of Motomasa, the Sogō and Yasutomi clans were in conflict. In 1523, Motomasa drove back Yasutomi Morikata. In 1526, in a battle against Sogō Kageshige, Motomasa received reinforcements from the Kagawa and Kōzai clans to defeat the enemy forces with ambushes and pincer attacks. On the provincial border of Sanuki, he routed Miyoshi Motonaga whose forces had come as reinforcements for the Sogō army. In 1532, Motomasa was attacked at Ikenouchi Castle by Sogō Kazumasa. Retainers from the Kabe clan raided the enemy base and injured Kazumasa, but Kazumasa calmly picked-up so he earned the nickname of “Demon Sogō.” Thereafter, a settlement was reached through the mediation of Hosokawa Harumoto, the deputy shōgun of the Muromachi bakufu.
In 1540, after conflicts with the Yasutomi clan intensified, Motomasa moved his base from Ikenouchi Castle to Hirune Castle. In the ninth month of the same year, a battle occurred in Shioki in Sanuki Province, but neither side prevailed. The conflict ebbed and flowed over the ensuing three-year period. In 1562, at the Battle of Kyōkō Temple, Motomasa, along with Sogō Kazumasa and Yasutomi Morikata, joined Miyoshi Nagayoshi and achieved victory.
After Motomasa’s demise, he was succeeded by his eldest son, Sangawa Motochika.