•WBC, WBA and IBF champion Charlo, 31, won Castano’s WBO belt in Los Angeles to unify the division.
•Charlo’s record now stands at 35 wins and just one defeat.
American Jermell Charlo became the first undisputed light-middleweight champion in history with a 10th-round knock out of Argentina’s Brian Castano.
It was the second fight between the pair, following a controversial draw last year.
WBC, WBA and IBF champion Charlo, 31, won Castano’s WBO belt in Los Angeles to unify the division.
“This is unbelievable. I gave it my all and that was the end result of it,” Charlo said.
After trading the opening rounds, Charlo started to get on top and sent his 32-year-old opponent to the canvas twice in the 10th.
First, a body shot was followed up with a left hook that put the previously undefeated Castano on the floor.
He struggled to his feet but a left to the head and another body shot finished Castano.
“I was pretty sure he was going to get back up because it was only the first knockdown but I knew it was over,” Charlo said.
Charlo’s record now stands at 35 wins and just one defeat.
He is only the seventh boxer to hold all four belts at the same time, joining the likes of Bernard Hopkins, Terence Crawford, Saul ‘Canelo’ Alvarez and Scotland’s Josh Taylor.
A gracious Castano paid tribute to his opponent, calling him “a champion”.
“We both were fighting back and forth. It was power back and forth, and then his right hand came over and stopped the fight. He’s a champion. He hit me. He got me,” he said.
“We showed that we are warriors. That’s the main thing. We have to feed our families.”