Artur Beterbiev stopped Oleksandr Gvozdyk to become the IBF, WBC and lineal light heavyweight champion.
Beterbiev recorded three knock downs in round 10 after a competitive fight in Philadelphia between the two undefeated title holders.
The pair traded heavy punches from the start with the Russian gradually wearing down Gvozdyk to record a 15th knockout in as many fights.
Gvozdyk was ahead on two of the three cards when the fight was stopped but Beterbiev landed the harder blows when it counted and is now motivated to win even more titles.
"This moment was my most important fight," Beterbiev said.
"I had big work to prepare for this fight. I was happy to end it.
"When I got this second belt, I got motivation. I was training so-so before, but from now on I was training very hard."