Samuel Pihlström ran in 3:40.59, a bit off his personal best of 3:33.47, which he set in Toruń, Poland earlier this winter.
The Swede was third for much of the last lap, but dropped to fourth place on the run - 33 hundredths of a second from bronze.
"That's the kind of way to lose it. I went out a little too far on the run and, yeah, I let (Adam) Spencer in on the inside," Pihlström said.
"I won't make that kind of mistake again."
Pihlström was expected to win his first senior championship medal at the World Indoor Championships in Toruń, Poland. He missed the chance at the 2024 European Championships when he fell in the heats and broke his wrist. At the Olympics, he did not advance from the heats.
Last year he finished sixth at the European Indoor Championships and then fifth at the World Indoor Championships. Last fall he finished eleventh at the World Outdoor Championships.
In this year's World Indoor Championships he ran a tactically very good heat, which he won. In the final the tactics held up almost the entire way. He led at the start when no one else wanted to pull, then was in third place all the way to the finish.
"If that tactic had worked, it would have been enough for a medal. I did what I could; there were three who were better today."
Spain's Mariano García won in 3:39.63. Isaac Nader of Portugal took silver and Adam Spencer of Australia bronze.
