Doi Toshikatsu (土井 利勝, April 19, 1573 – August 12, 1644) was a top-ranking official in Japan's Tokugawa shogunate during its early decades, and one of the chief advisors to the second Tokugawa shogun, Hidetada.
The adopted son of Doi Toshimasa, Toshikatsu is generally believed to be the biological son of Mizuno Nobutomo, though there are some who claim he was an illegitimate son of shogun Tokugawa Ieyasu. He served the shogunate as advisor to shogun Tokugawa Hidetada for many years, and played an important role in communicating and overseeing the enforcement of shogunal policy across the country; Doi also helped effect trade and diplomatic relations between Japan and the Thai Kingdom of Ayutthaya. He lost much of his influence and power upon Hidetada's death in 1632. Six years later, however, Doi became one of the first to be appointed to the newly created post of Tairō (Great Elder), and was made daimyo (feudal lord) of Koga Domain in Shimōsa Province, with a revenue of 160,000 koku.