Mosley (right) dominated the contest with 108 power punches
Shane Mosley pulled off a stunning ninth-round stoppage of Mexican Antonio Margarito in Los Angeles to claim the WBA welterweight title.
American Mosley, 37, dominated the contest with his slick hand and foot speed and came close to stopping Margarito in the eighth.
And when Mosley unleashed another flurry of combinations in the next round, referee Raul Caiz stepped in.
Mosley, who has 46 wins from 51 fights, is now a four-time world champion.
He said: "It was my strategy, my focus and my game-plan that won it. I knew it was going to be a tough fight but it was a great plan."
Margarito was coming off an impressive victory over the previously unbeaten Puerto Rican Miguel Cotto six months ago and was a strong betting favourite.
However, and despite his advancing years, Mosley used his superior speed from the beginning never allowed the 30-year-old Margarito into the contest.
Margarito, who fell to 37 wins and six defeats, did not win a single round on one judge's scorecard, won one on another and two on the third.
Watched by a sellout crowd of 20,820 at the Staples Center, both fighters engaged at close quarters in the opening round with Mosley holding a slight advantage.
Margarito upped the tempo in the second round, forcing Mosley to backpedal, briefly pinning him to the ropes and landing with a hefty uppercut.
However, Mosley stopped Margarito in his tracks with a spearing right jab in round three and landed with some telling blows to the body in round four.
In the fifth round, Margarito made contact with a few body blows before he was caught by a solid left hand by Mosley, who continued to control the pace of the fight.
Mosley dominated the next two rounds, pummelling Margarito almost at will with clever combinations and jerking Margarito's head back with an overhand right just before the bell.
The American came close to ending the fight in the eighth, landing with a crunching jab to Margarito's body before connecting with a crushing uppercut to the Mexican's head.
And he maintained his dominance with a flurry of combinations early in the ninth before Caiz called a halt to proceedings 43 seconds into the round.
"I feel OK," said Margarito, who had never been stopped. "I was just getting caught over and over."
Mosley, a former world champion at lightweight, welterweight and light-middleweight, may now opt to defend his crown against Cotto, who outpointed him when they met in November 2007.