In the winner's round, Wennmalm was responsible for a triple vault with four screws, which was decisive in the final round.
In total, he landed on 44.7 points out of a maximum of 50.0.
Second was Aleksi Sainio, Finland, on 44.0 and third was Swiss Robin Steiner on 43.0.
The championship, the first ever, took place in London last week.
- I knew it would be great to win the first championship.
People will remember this, something that changed the industry.
It means a lot for the sport.
It's cooler to be part of the sport's development than to win the gold, he says.
Watch Sebastian Wennmalm's decisive number in the player above.