The 20-year-old - who has signed a one-year contract with the option of a further year - left Hibernian at the end of last season.
The striker spent time out of loan at Alloa Athletic last season but has now returned to the Scottish top flight. "We hope that we develop him into the player that we believe he can be," said Tommy Craig, the St Mirren manager.
Caldwell's former club also filled a gap in their attack, with Farid El Alagui signing a two-year contract at Hibs. "He bought into my vision for the club," said Alan Stubbs, the Hibs head coach.