The bigger MacDonald used his size and reach to blunt Saffiedine's kickboxing in a strategic fight that featured isolated bursts of action in the first two rounds.
But it turned in the third. MacDonald (18-2) clipped Saffiedine with a shot to the back of the head and then put him down with another punch, finishing him off with blows on the ground before referee Herb Dean stepped in one minute 28 seconds of the round.
Saffiedine was left dazed and bloody on the canvas.
"I want that title shot," said MacDonald.
The 25-year-old MacDonald, a native of Kelowna, B.C., who fights out of Montreal, came into the bout ranked No. 2 among 170-pound contenders while Saffiedine (15-4) was ninth.
Johnny (Bigg Rigg) Hendricks defends his welterweight title against No. 1 contender (Ruthless) Robbie Lawler at UFC 181 on Dec. 6 in Las Vegas.
MacDonald has won three straight and eight of his last nine, losing a split decision to Lawler at UFC 167 last November. Saffiedine had won his last five bouts.
Saffiedine, 28, walked out first. The American-based Belgian wasted little time, raising his arm in the air as he entered the arena.
Then came MacDonald, to cheers and the pounding sound of Metallica's "St. Anger." As he was introduced, the stone-face Canadian stared across the cage at Saffiedine.
In a cautious opening, both fighters fired kicks. MacDonald took the centre of the cage and slowly stalked Saffiedine, taking him down briefly with 90 seconds remaining.
MacDonald scored with strikes early and late in the second. Saffiedine answered in the middle of the round with some kicks that landed with a slap on MacDonald's leg.
Saffiedine's corner had a go at him after the second round. The Belgian closed the gap and launched more kicks in another cat-and-mouse round before MacDonald saw his opening and pounced.
"I was just going with the flow .. I wasn't trying to force anything and it worked out," said McDonald, a 4-1 favourite.
The UFC's first visit to Atlantic Canada drew a loud crowd of 10,782 at the Scotiabank Centre.
They had plenty to cheer about as Canadian fighters won seven of their nine bouts.
In the co-main event, Georgia-based Brazilian bantamweight Raphael Assuncao used his speed to earn a unanimous 30-27 decision over Bryan (Kid Lightning) Caraway for his seventh straight UFC win. The five-foot-five Assuncao, the last man to beat current 135-pound champion T.J. Dillashaw, came into the fight ranked No. 4 among bantamweight contenders while Caraway was No. 10.
Caraway, damaged during the fight by an Assuncao knee to the face, kept coming but had no answers for Assuncao.
The crowd roared from announcer Bruce Buffer's opening words and local flyweight Chris (The Greek Assassin) Kelades authored a feel-good story with his fists just three fights into the 12-bout card.
Kelades, from nearby Cole Harbour, only found out Monday he was fighting as a late replacement (for the pneumonia-stricken Louis Gaudinot) on the card that he had bought tickets to see.
He came into the bout against Ireland's Patrick (The Hooligan) Holohan as a 3-1 or 4-1 underdog. But the 33-year-old Kelades (8-1) emerged victorious, winning a unanimous 29-28 decision over the previously undefeated Holohan (10-1-1).
In addition to the win, the only Nova Scotian on the card made history by becoming the first Canadian 125-pounder to fight in the UFC.
Both Kelades and Holohan earned US$50,000 bonuses as their bout was named fight of the night.
There was a more painful end to the UFC debut of Kelowna welterweight Matt Dwyer.
New Jersey-based Russian Albert (Einstein) Tumenov (14-2) floored the six-foot-four Dwyer with two resounding kicks to the head before the fight was stopped after 63 seconds. Dwyer (7-2) paused to watch the replay of the kicks as he made his way from the cage, an ugly mouse under his eye. He was taken to hospital after the bout to be checked out.
Mitch Gagnon of Sudbury, Ont., continued his impressive rise up the bantamweight ranks with his fourth straight UFC win, submitting late replacement Roman Salazar via rear-naked choke at 2:06 of the first round.
Gagnon, a five-foot-five buzzsaw, took Salazar's back within the first two minutes. Salazar (9-3) struggled to his feet with Gagnon attached like a clam but fell back down and tapped. Salazar, a Tucson cable technician, was Gagnon's third opponent after two others dropped out through injury.
It was the 10th first-round finish for Gagnon (12-2) and his 11th win by submission. He lost his UFC debut to Caraway in July 2012 but still earned a fight of the night bonus.
Elsewhere on the card, the two winners of "The Ultimate Fighter Nations" reality TV show remained undefeated.
Toronto middleweight Elias (The Spartan) Theodorou (11-0) used a strong third round, complete with several takedowns, to win a unanimous 29-28 decision over burly Brazilian Bruno (Carioca) Santos (14-2) in a grinding fight.
TUF Nations welterweight winner Chad (The Disciple) Laprise (10-0), moving down to lightweight, had too much for Cuban-born Miami-based Yosdenis (The Pink Panther) Cedeno (10-4) en route to a workmanlike 30-27 unanimous decision.
Cedeno's best moment in Halifax was when, as a proud father, he excitedly waved an ultrasound picture as he weighed in Friday.
Nordine Taleb, born in France but fighting out of Montreal, won a split decision (28-29, 29-28, 30-27) over China's Li (The Leech) Jingliang in a close battle between big welterweights. Taleb, who fought as a middleweight on "The Ultimate Fighter Nations" show, was supported by his cornermen as he limped out of the ring favouring a broken foot.
Both men would be well-served to avoid mirrors for a few days.
Charlottetown lightweight Jason Saggo (10-2) lost a split decision (28-29, 29-28, 28-29) to Paul (The Irish Dragon) Felder (9-0). Montreal lightweight Olivier Aubin-Mercier (6-1) submitted Jake (The Librarian) Lindsey (9-2) by inverted triangle choke at 3:22 of the second round.
Both MacDonald and Aubin-Mercier went home with an extra $50,000 for the performance of the night.
(Detroit Superstar) Daron Cruickshank (16-5) won a 30-27 decision over lightweight Anthony (The Assassin) Njokuani (16-9 with one no contest) in the featured preliminary bout. Cruickshank earned cheers when he picked up Njokuani and ran the length of the cage with him slung over his shoulder before driving him into the canvas.
Njokuani later apologized via Twitter. "My head wasn't in the game and it cost me the fight."
The Halifax crowd was into it from the get-go, cheering Buffer before a punch was thrown. And they got action early as American-based Brazilian bantamweight Pedro (The Young Punisher) Munhoz submitted former Oklahoma State wrestler Jerrod (J-Reazie) Sanders in 39 seconds in spectacular fashion.
Stuffing a takedown, Munhoz (12-1) locked on a guillotine choke. Sanders (14-3) fought his way to his feet, with Munhoz looked around his head like an alien, and then smashed Munhoz to the ground in a bid to dislodge him. Munhoz hung on and Sanders tapped.
The Halifax card featured athletes from 10 different birth countries: Belgium, Brazil, Canada, China, Cuba, France, Ireland, Nigeria, Russia and the U.S.
Another 24 fighters competed in the cage at Stockholm's Ericsson Globe Arena on another UFC card earlier Saturday. American welterweight Rick (The Horror) Story upset highly touted Gunnar (Gunni) Nelson via split decision in the main event in Sweden.
Halifax is the seventh Canadian city to host a UFC show, joining Calgary, Montreal, Quebec City, Toronto, Winnipeg and Vancouver. Saturday marked the UFC's 17th Canadian show since 2008.
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