Yoshisada was a son of Arima Haruzumi. He ruled the Shimabara area of Hizen but was steadily weakened in wars with the Ryûzôji. He was defeated by Ryûzôji Takanobu in 1563 and in 1570 sent troops to assist the Ôtomo in their own struggle with Takanobu (which culminated in the Battle of Iyama). He retired in 1571 in favor of his younger brother Harunobu.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005