It was a high-tempo, take-no-prisoners fight with St-Pierre pushing the pace. The first two rounds went to GSP but Condit threatened briefly in the third. St-Pierre consolidated his lead with takedowns as the fight wore on.
Ultimately GSP's wrestling skills and ability to resist Condit's submission attempts on the ground won the day in what was an entertaining fight. St-Pierre controlled the contest.
The two battered fighters shook hands and embraced when the fight ended. The judges scored it almost a shutout for St-Pierre: 49-46, 50-45, 50-45.
St-Pierre had not fought since April 2011 as the result of reconstructive knee surgery following an injury suffered in training. Condit won the interim title during his absence, defeating Nick Diaz in February.
There were questions going into the fight. Would it be business as usual for St-Pierre in his seventh title defence? Or was the returning 170-pound champ ripe for the taking from a difficult opponent who seemed to be peaking at the right time.
St-Pierre (23-2) proved willing and able.
"People talk about ring rust. I definitely know what it is now," said St-Pierre. "Credit to Carlos, he definitely gave me my toughest fight."
St-Pierre said his knee was fine throughout the fight. He arrived to the press conference afterward in a suit with an icebag on his head, along with a thick lip and an assortment of bumps and bruises. Condit's face was also marked up.
The GSP win likely means a super-fight showdown with middleweight (185-pound) champion Anderson Silva. The 37-year-old Brazilian met the media before the card started, declaring his interest in meeting GSP somewhere between their two weight classes.
"I need to take some vacation and think about it," St-Pierre said of the super-fight.
Said Condit: "His top game was very, very good. He was able to hold me down better than I expected. I feel like I did pretty well ... but I need to go back to the drawing board."
Silva was watching cageside, next to former light-heavyweight champion Lyota (The Dragon) Machida.
Condit walked to the cage to boos and the harsh sound of Rage Against the Machine.
St. Pierre followed in his traditional martial arts gear and headband, accompanied by a tsunami of cheers from the Bell Centre crowd and booming music of French rapper Fababy. The Maple Leaf flashed on the arena video screens.
The arena went wild as he entered the cage, with St-Pierre celebrating his return to action with a cartwheel. The chants of GSP started before the fighters were introduced.
Saturday's event had a crowd of 17,249, for a gate of $3.143 million.
GSP stalked the taller Condit from the get-go as the crowd sang "Ole, Ole, Ole." The first takedown came within two minutes, with St-Pierre looking to score from above in Condit's guard. Condit, comfortable fighting off his back, looked cool but took some damage.
Condit (28-6) was cut above the right eye by an elbow near the end of the round.
St-Pierre opened with strikes and kicks in the second, calmly cutting down the cage as he closed on Condit. He greeted an attempted flying knee with a punch to the face.
Condit swung away, trying a spinning backfist that missed. St-Pierre fired with the jab then took him down again with two minutes remaining. He passed guard and went to work, bloodying Condit up some more.
Condit tried to fight back from the bottom but was taking punishment. The two got up and exchanged blows as the round ended, with a bloody Condit raising his arms. GSP's face was also showing signs of wear.
Condit knocked St-Pierre down with a head kick early in the third and swarmed him. GSP weathered a dangerous storm and finally got back up, his face leaking blood. He then dumped Condit on the ground and won the rest of the round.
"I thought I had him there," Condit said of the kick.
Both men needed work by the cutmen after the round.
Another GSP takedown opened the fourth round as the two wrestled on a bloody canvas that looked like something from "CSI: Las Vegas." Condit tried for a submission off his back, but St-Pierre fought him off and started hitting him again.
Condit came forward in the final round, in search of some answers. St-Pierre stood back and counter-punched. Then he took Condit down again, and the fight ended with GSP on top amidst the din of the Bell Centre.
The 31-year-old St-Pierre came into the fight having won nine straight and not having lost since April 2007 when he was upset by challenger Matt (The Terror) Serra in his first title defence.
In the co-main event, Johny (Bigg Rigg) Hendricks knocked out Martin (The Hitman) Kampmann in 46 seconds in a battle of welterweight contenders. It was a slightly longer replay his 12-second KO of Jon Fitch.
Hendricks threw a right then nailed Kampmann (20-6) with a left, sending him flying backwards. He hit the Las Vegas-based Dane one more time on the ground before the referee stepped in.
Hendricks (14-1) mimed holding the championship belt around his waist as he celebrated this fifth straight win.
"Please, please give me a shot at the belt," said Hendricks, a former two-time NCAA champion wrestler with heavy hands.
It was a night of perspiration more than inspiration with one KO, seven decisions, two submissions and a disqualification prior to the main event.
"The co-main event and main event saved the night,' said UFC president Dana White.
The crowd roared every time there was a glimpse of GSP ahead of the main event. Condit was clearly on hostile ground.
The 28-year-old from Albuquerque, N.M., was booed as he was shown on the video screens entering the arena. The crowd went wild at GSP's televised entry, especially when the champ faked a limp before grinning and giving a thumbs-up.
There were eight Canadians on the card, not counting Montreal-based French middleweight Francis Carmont — one of five Tristar Gym fighters in action.
There would have been nine but Calgary middleweight Nick (The Promise) Ring was forced to pull out earlier in the day due to illness. His bout with American Costa Philippou was scrapped.
Montreal's John (The Bull) Makdessi looked impressive in defeating Sam (Hands of Stone) Stout of London, Ont., in an all-Canadian lightweight bout.
Makdessi (10-2) used a smart game plan — and stinging left jab — to pick away at Stout (19-8-1) in a clinical 30-27, 29-28, 30-27 win.
The other Canadians faced foreign opponents on the night, going 3-2 prior to the main event.
Veteran Montreal middleweight Patrick (The Predator) Cote won by first-round disqualification over Italy's Alessio (Legionarius) Sakara.
The fight was stopped at 1:26 after Sakara floored Cote with elbows to the head and then nailed the dazed Canadian — who was on his knees — with hammer-fists, of which eight straight were illegal blows to the back of the head.
The win was eventually given to Cote (19-8) after consultations in the cage.
"I was expecting a war with Alessio and that's exactly what he gave me but there are rules for a reason," said Cote.
Sakara (19-10 with one no contest) apologized after the fight, saying he was caught up in the moment.
Featherweight Mark (The Machine) Hominick of Thamesford, Ont., lost his fourth straight, via 29-27, 30-26, 29-28 decision to Pablo (The Scarecrow) Garza.
It was a wild first round with Hominick hurting the taller Garza before Garza (13-3) carved open Hominick's face below the left eye.
"Great 1st round!!!!!!." tweeted UFC boss Dana White.
The second round was Garza's as he put Hominick (20-12) on his back and punished him. Hominick's face started to show damage and things went from bad to worse as the relentless Garza controlled the Canadian on the ground.
Brazilian lightweight Rafael Dos Anjos won his third straight, defeating Mark Bocek of Woodbridge, Ont.
Bocek, bleeding in the third round from strikes, couldn't get the fight to the ground to use his jiu-jitsu, with the Brazilian dictating the bout. Dos Anjos (18-6) won a unanimous 30-27 decision and Bocek (11-5) left the cage with a towel pressed to his face.
Carmont (20-7) won a split (29-28, 28-29, 29-28) decision over (Filthy) Tom Lawlor in a drab contest that drew boos both for the action and the decision. Neither fighter did much but Lawlor (8-5 with one no contest) seemed to be trying more.
"Who cares! They both lost that fight," tweeted White.
Montreal bantamweight Ivan (Pride of El Salvador) Menjivar pulled off a slick belly-down armbar to defeat Azamat Gashimov, an American-based Russian, by submission at 2:44 of the first round.
Gashimov (10-2) took Menjivar (25-9) down and paid for it. Gashimov's face was contorted in pain as he tapped out in his UFC debut, his arm bent at an impossible angle.
Featherweight Antonio (Pato) Carvalho (15-5) of Ajax, Ont., won a split (29-28, 28-29, 29-28) decision over former Strikeforce contender Rodrigo Damm (10-6) in a bout dominated by savage leg kicks
Lanky French light-heavyweight Cyrille (The Snake) Diabate (20-8-3) showed off his ground skills in choking out Chad (The Grave Digger) Griggs — a full-time Tucson firefighter — at 2:24 of the first round.
Welterweight Matt (Deep Waters) Riddle (7-3 with one no-contest) won a unanimous decision over England's John (The One) Maguire (18-5).
Darren (The Damage) Elkins (16-2) ran his winning streak to four at featherweight with a unanimous decision over a game (Super) Steven Siler (21-10).
St-Pierre and Condit were awarded the fight of the night bonus, Hendricks earned the knockout bonus and Menjivar had the best submission. That's an extra $70,000 for each fighter.