- Born: 1562
- Died: 1615
- Titles: Iga no kami
- Distinction: Toyotomi retainer
- Japanese: 筒井 定次 (Tsutsui Sadatsugu)
Sadatsugu was adopted by his uncle Junkei around 1571 and in 1578 married one of Oda Nobunaga's daughters, Hideko (d.1632). In 1584 he inherited Ueno castle in Iga province and a 120,000 koku domain in Iga, Ise, and Yamashiro provinces and assisted in Hideyoshi's Kii Campaign against the Negoro-ji. He supported the Tokugawa during the Sekigahara Campaign and led some 2,800 men at the forefront of the Battle of Sekigahara. He was deprived of his lands for poor administration in 1608 and exiled to Iyo province. He was later made to commit suicide after being accused of colluding with the defenders of Osaka castle.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005