Blackburn captain Jason Lowe remains calm over rumours star striker Rudy Gestede will be sold this month.
Samuel Eto’o has completed a medical with Italian club Sampdoria.
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.”