Miguel Cotto stopped fellow Puerto Rican Carlos Quintana in the fifth round to claim the vacant WBA welterweight title in Atlantic City.
Cotto won the WBA title vacated by Ricky Hatton
Cotto floored Quintana twice in round five and his bloodied opponent was stopped from coming out for the sixth.
It was the former WBO light welterweight king's first bout at 147lb and he improved his record to 28 wins, 23 knockouts and no defeats.
The WBA belt was vacated by Britain's Ricky Hatton earlier this year.
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Cotto was coming off six defences of the light welterweight title he captured in 2004 and seemed to carry his power and speed up to the higher weight.
Cotto came out bombing from the opening bell and mercilessly pounded Quintana, who entered the fight 23-0 with 18 knockouts.
"I felt very strong at this weight and every punch I throw is hard," Cotto said. "I felt I could do anything I wanted to do."
On the same bill, Antonio Margarito unanimously out-pointed Ghana's Joshua Clottey to hold on to his WBO welterweight title.
Mexico's Margarito, 28, took control in the middle rounds and pummelled Clottey in the eighth and the ninth to pull way ahead on all three judges' scorecards.
Afterwards, Margarito, who has 34 wins and four losses, said he would like to face Cotto next in a unification battle.