Marquez has been mentioned as a future Ricky Hatton opponent
Juan Manuel Marquez of Mexico claimed the vacant WBA and WBO lightweight belts with a ninth-round knockout of American Juan Diaz in Houston, Texas.
Marquez opened a cut over Diaz's eye in the eighth round, then knocked him down twice in the ninth.
Marquez first dropped Diaz with a four-punch combination before finishing the fight off with a right uppercut.
Marquez, 35, improves to 50 wins, four losses and one draw, while Diaz, 25, drops to 34 wins and two defeats.
Diaz lost for the second time in three fights. His other defeat was to Nate Campbell, who was stripped of his WBA and WBO titles after he failed to make weight for his own defence two weeks ago.
Marquez, a former featherweight and super-featherweight world champion, was fighting at 135lb for only the second time in his career.
"I was ready to leave my heart and my soul in that ring," Marquez said. "It was a great victory for me. It is one of the most important victories of my career."
Marquez also became the latest fighter to call out Floyd Mayweather Jr, who was considered the pound-for-pound king before he retired last summer.
"Who wouldn't want to see that fight?" Marquez said. "I'll fight him at 140lb (light-welterweight), maybe a little above that. I want to fight the best, the very best."
Marquez has been trying for months to land one more fight with Filipino Manny Pacquiao, who will face Ricky Hatton on 2 May.
Marquez fought Pacquiao to a draw in May 2004, then lost a close decision in the rematch last March. Many observers thought Marquez should have won both.
"I've been trying to get a third fight with Manny Pacquiao, but he doesn't want to fight me," Marquez said. "So I'm calling out the best, and that's Floyd Mayweather."
Diaz won the early rounds on Saturday, repeatedly backing Marquez onto the ropes before hammering him with quick combinations.
He snapped back Marquez's head with a hard left in the third, but Marquez fought out of the corner and connected with a right uppercut in the centre of the ring.
Marquez altered the momentum when he cut Diaz, a similar situation to Diaz's bout against Campbell last March, when a cut seemed to change the entire fight.
"I got caught with a good punch. There was nothing I could do," he said. "I couldn't see out of my right eye."
Headlining the undercard, Chris John retained his WBA featherweight title by battling hometown hero Rocky Juarez to an entertaining draw.