Masatsuna MATSUDAIRA became an adopted son of the Nagasawa Matsudaira family, which was said to be the Nitta clan branch of the Kawachi Genji (Minamoto clan), which in turn belonged to the House of Tokugawa; since then, the clan has been known as the Okochi Matsudaira family.
松平正綱の代に、徳川氏一族の河内源氏で新田氏流とされる長沢松平家の養子となり、それ以後は大河内松平家と言う。