Tokugawa Tadanaga (徳川 忠長?, 1606 – January 5, 1634) was a Japanese daimyo of the early Edo period. The son of the second shogun Tokugawa Hidetada, his elder brother was the third shogun Tokugawa Iemitsu.
Often called Suruga Dainagon (the major counsellor of Suruga), Tadanaga was born in 1606. His birth name was Kunichiyo (国千代). The date of his birth is uncertain, and is variously given as May 7, June 1, and December 3. Blessed with military and intellectual prowess and a generosity of spirit, he received support from his mother, Oeyo (or Sūgen'in), who favored him over her other son Takechiyo (the future Iemitsu) to become the third shogun. Just after the death of their father shogun Hidetada, Iemitsu accused his brother, already under house arrest in Kofu, of insanity, stripped him of all possessions and offices, leaving him to commit seppuku.
- 1606: Born to Hidetada and Sūgen'in
- 1624: Appointed daimyo of Suruga, Tōtōmi, and Kai Provinces, with a rating of 550,000 koku (see Kōfu Domain)
- 1626: Kills a retainer and commits other acts of violence
- 1631: Placed under house arrest in Kofu
- 1632: Stripped of office
- 1633 (or 1634): Commits seppuku under arrest in Takasaki
- Beatrice M. Bodart-Bailey, "The Dog Shogun" p. 14