Chelsea striker Fernando Torres is understood to be set for a return to Atletico Madrid, according to Sky sources.
Mark Hughes insists Stoke will not be looking to make any major additions in the January transfer window.
The 20-year-old has been on trial with the Buddies during pre-season training and has done enough to earn a deal from manager Tommy Craig.
Craig told his club’s official website: “We’re delighted to have Ross on board.
“He is still on his way to fulfilling the potential that he has shown for quite some time and we hope that we develop him into the player that we believe he can be.”
Caldwell, who made his debut for Hibs in December 2011, said: “I am delighted to have joined St Mirren. Having been at the club now for a couple of weeks I have thoroughly enjoyed every minute here and already feel part of the set up.
“I now want to repay the faith shown in me in the season ahead.”