Buddies boss Danny Lennon moved quickly to make the capture after summer signing Grant Adam was injured while playing for the under-20 side on Tuesday.
Adam is expected to be sidelined for up to four months and Saints have recruited Smith until January.
The 26-year-old was a product of the St Mirren youth system before going on loan to Dunfermline midway through the 2009-2010 season and making the move permanent.
Lennon told the club's website: "Chris Smith is no stranger to St Mirren Football Club.
"We got a bit of bad news about Grant Adam's injury so to keep the depth in the squad strong we felt it important to go out there and see to that.
"Chris looks in great condition and under Jim Jefferies at Dunfermline he played well at the tail end of last season and we'll look to kick him on again."
Lennon added: "Grant will be out of action for the best part of three to four months as he has damaged his posterior cruciate ligament and has a hairline fracture on his tibia. It is a...