Florian Wellbrock also won the gold medal in open water swimming over five kilometers at the World Championships in Budapest.
The 24-year-old beat his long-time Italian competitor Gregorio Paltrinieri on Monday in the 26.7-degree warm Lupa Lake.
Bronze went to Wellbrock's training partner Mykhailo Romanchuk from Ukraine.
Wellbrock struck after 52:48.8 minutes.
His lead over Paltrinieri was 3.9 seconds.
For frequent starter Wellbrock it was already the fourth medal in the fourth competition at the World Championships and the second title.
Born in Bremen, who trains in Magdeburg, he won silver in the 800-meter freestyle and bronze in the 1,500-meter freestyle.
On Monday he also secured the gold medal for the German open water relay team with a strong final sprint.
In the five-kilometer race, which the open-water Olympic champion has added to his program compared to the last World Championships, Wellbrock swam in front from the start.
About halfway through, he was leading a five-man lead group.
On the final lap he pulled away with Paltrinieri and won easily.
Wellbrock followed in the footsteps of Thomas Lurz.
The record world champion had triumphed in 2011 as the last German open water swimmer on this route at a World Cup.