When the blood lineage of the Wakebe clan was discontinued after the former domain lord Yoshitake WAKEBE died young without heir, Nobumasa was adopted to succeed to the Wakebe clan in 1667 because he was a relative of Yoshitaka on the mother's side.
寛文7年（1667年）、先代藩主の分部嘉高が嗣子なくして若死にして分部氏の血統が断絶すると、嘉高の母方の縁戚に当たると言うことから、その養嗣子となって後を継いだ。