Algeria's Taoufik Makhloufi left the rest of the field trailing to win the first global title of his career.
Makhloufi, 24, finished in a time of three minutes 34.08 seconds, racing away from American Leonel Manzano who took silver in 3:47.79, with Moroccan Abdalaati Iguider in bronze.
"It was a funny race. I'm really surprised those guys let it run at such a slow pace when they knew Taoufik Makhloufi had such a strong finish. They played into his hands."
Makhloufi was reinstated in the Games on Monday, having been disqualified
for not trying in his 800m heat.
Defending champion Asbel Kiprop was last, apparently hampered by injury.
Makhloufi, a semi-finalist at last year's World Championships in Daegu, set off quickly and led alongside Iguider after the first lap, before falling back to sixth after the second lap.
He was battling hard with Kenya's Kiprop, but as the Beijing champion's challenge faded, the Algerian positioned himself well to attack the frontrunners in the closing stages.
He took the lead in the back straight and, with around 200m to go, accelerated easily away from his nearest rivals to race unchallenged towards the finishing line.
Makhloufi, who won his 1500m semi-final impressively on Sunday, had been forced to race in the 800m after Algeria failed to withdraw him before the deadline.
He jogged through about 200m of his heat before quitting, and was then excluded from the Games for "not providing a bona fide effort".
However, after inspecting medical evidence, the International Association of Athletics Federations decided Makhloufi could race in the 1500m final.