Toshima Yasuaki was the younger brother of Toshima Yasutsune, a Muromachi-era samurai who died in the Battle of Egota-Numabukurohara fighting against Oota Doukan.

Toshima Yasuaki, along with his brother and Toshima Clan Head Toshima Yasutsune, came to the aid of Nagao Kageharu when he rebelled against the Uesugi clans. He raised an army at his main castle of Hiratsuka, but was attacked by Oota Doukan, who set fire to the lands around the castle. Yasutsune came to his aid with troops from Shakujii Castle and Nerima Castle, whereupon Doukan immediately drew back until he met troops from Uesugi Tomomasa and Chiba Yoritane sortieing from Edo Castle. The two sides met in the Battle of Egota-Numabukurohara in what is now Nerima City.

The battle resulted in a great loss for the Toshima clan; of their 200 cavalry, 150 were killed, including Yasuaki. Afterwards, Doukan laid siege to Yasutsune at Shakujii Castle, which fell in 28 days. Yasutsune escaped, returned with another army, was defeated again, and became lost to history. The Toshima Clan perished.