Vasyl Lomachenko outpointed Gary Russell Jr in Carson, California to win the vacant WBO featherweight title in only his third professional fight.
Lomachenko tied the record for fewest fights to win a world title, set by Thailand's Saensak Muangsurin in 1975.
In March, Ukraine's two-time Olympic champion, 26, lost his first title shot against Mexican veteran Orlando Salido.
American Carson had success in the early stages but Lomachenko grew into the bout and was well on top late on.
Two judges scored the fight 116-112 for Lomachenko while the third scored it a 114-114 draw.
Lomachenko was arguably the best amateur boxer of his generation, winning Olympic gold medals in 2008 and 2012 and winning 396 of his 397 unpaid fights.
But in his second pro fight Lomachenko lost a split decision to Salido, who failed to make the weight and was stripped of his WBO belt.