The defender joined the Pirates - rivals of his former club Bristol City - in November on a three-month loan, which would have expired at the end of the month.
He has now signed a 2½-year contract with the League One side, transferring for an undisclosed fee.
The 26-year-old has made 7 appearances for them in the league and 2 in the FA Cup, the latest of those as a goalscoring midfielder in Saturday's draw with Fulham.
He's the third player to leave Hull City in the January transfer window, following John Welsh's loan move to Chester and Steve McPhee's transfer to Blackpool.