Rashid Ramzi delivered a fast finish to claim the 800m and become the first athlete to seal an historic double at the World Championships. The Bahrain runner, who had already won the 1500m, came strongly off the bend to cross the line in a personal best of one minute 44.21 seconds in Helsinki.
Olympic champion Yuriy Borzakovskiy took silver after relying too heavily on his sprint finish.
Kenya's William Yiampoy edged out team-mate Wilfred Bungei for bronze.
Algeria's defending champion Djabir Said-Guerni tried to push his way into contention but came home in fifth.
Ramzi's success in Finland comes just a year after he failed to get beyond the semi-finals at the Olympic Games.
"I learned from my mistakes in Athens," said the 25-year-old.
"These results are simply down to me preparing better."
Meanwhile, Borzakovskiy was left rueing his mistimed tactics after he started his sprint for the line too late to catch Ramzi.
"I lost the race at the 600m mark," said Borzakovskiy.
"I was closed in behind (Mehdi) Baala and lost my rhythm."