- Birth: 1527
- Death: 1596
- Title: Saemon no Kami
- Distinction: Tokugawa retainer
- sons: Ietsugu, (Honda) Yasutoshi
- Japanese: 酒井 忠次 (Sakai Tadatsugu)
Tadatsugu was one of Tokugawa Ieyasu's most notable commanders. After Ieyasu split from the Imagawa clan after 1560, Tadatsugu (a vocal supporter of the break) was given command of Yoshida castle, which guarded the coastal road way into Mikawa from Tôtômi. At the Battle of Mikatagahara (1573) he secured the Tokugawa's right flank, and saw his command badly mauled by the attacking Takeda when the units (those sent by Oda) around him fled. At Nagashino he requested permission to lead a night attack on the Takeda camp, which he accomplished (along with Kanamori Nagachika) to good result. During the Komaki Campaign, he was dispatched to turn back a Toyotomi move against Kiyosu led by Mori Nagayoshi, and was successful. At the time of the Odawara Campaign (1590) he accompanied Tokugawa Hidetada (Ieyasu's hostage to Toyotomi Hideyoshi) to Kyoto. When the Tokugawa were afterwards moved to the Kanto, Tadatsugu recieved a 50,000-koku fief at Takasaki (Kôzuke Province). Despite Tadatsugu's high rank, some believe that Ieyasu never forgave him for an incident in 1579: while making a diplomatic visit to Oda Nobunaga, Tadatsugu was confronted with allegations that Ieyasu's son Nobuyasu was plotting against the Oda - no friend of Nobuyasu himself, Tadatsugu made no attempt to refute the charges (and Nobuyasu was later made to commit suicide). Ieyasu afterwards chided him for not making some effort to defend his son's honor.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005