Since his older brother Nobutoshi MATSUDAIRA, who had originally been expected to succeed to the position of family head, died in 1865, Nobumasa was adopted by Nobuyoshi MATSUDAIRA, and in 1866 he succeeded to the position, becoming the eighth lord.
はじめ家督は兄の松平信敏が継ぐ予定だったが、信敏が慶応元年（1865年）に死去したため、信正が松平信義の養子となって、慶応2年（1866年）に家督を継いで第8代藩主となる。