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 big blow for Grant and a big blow for the club.
"While Grant is on the mend we have reacted to it and we still give ourselves very good depth in the squad by signing Chris."
Meanwhile, Lee Mair expects former club Dundee to go all out to show they deserve their place in the SPL when they host the Buddies tomorrow.
The Dark Blues - Irn-Bru First Division runners-up - replaced troubled Rangers in the top-flight and earned a credible goalless draw at Kilmarnock in last week's opener.
Mair said: "It's going to very tough, especially with it being their first home game back in the SPL.
"It's going to be quite hostile and I imagine they'll have lots of fans so hopefully we can bring a few through as well and it's a good game.
"These boys have got something to prove by being back in the SPL and showing they are capable of playing there.
"They didn't win the league so people are maybe saying they don't deserve to be in the SPL so they'll have that between their teeth and will have something to prove."
The defender added: "I came through the youths at Dundee and was there for eight years so I know it's a massive club.
"They've had ups and downs and been in administration twice so hopefully now that they're back in the SPL, they can build on that.
"I've got a lot of great memories and a great feeling for the place - but hopefully it will be three points for us on Saturday."