Gennady Golovkin beat David Lemieux in an eighth-round stoppage to add the IBF middleweight title to his WBA belt.
The Kazakh fighter, 33, outclassed his Canadian opponent to win his 34th consecutive bout with a dominant performance in New York.
Golovkin had sent Lemieux staggering with a vicious left hook in the fourth round before knocking the 26-year-old to his knees in the fifth.
"I am happy I gave my friends and fans a big show," he said.
"My goal is to have all the belts in the middleweight division. I want to break a new story."
The victory puts Golovkin in line for a possible meeting with the winner of the 21 November fight between Puerto Rico's WBC middleweight champion Miguel Cotto and Mexican Saul Alvarez.
"He'll be at the fight and he'll be looking to fight the winner," promoter Tom Loeffler said. "Whoever wins will clearly be at the top of the sport, as Gennady is himself."