"Sugar" Shane Mosley beat Luis Collazo on a unanimous points decision to lift the WBC interim welterweight title in Las Vegas on Saturday.
Mosley improved his record to 44-4, with 37 KO
He was scored 118-109 118-109 119-108 and put Collazo down in the 11th round.
Collazo, who also lost his IBF belt to Britain's Ricky Hatton, was given just one round - the second - by the judges.
Mosley will progress to be full title holder when Floyd Mayweather Jr steps up to meet Oscar De La Hoya for the light middleweight crown in May.
Mosley previously held the full WBC welterweight crown from 2000-02 and also held titles at lightweight and light middle.
And having fought primarily as a light middleweight since 2002, the 35-year-old revealed he will almost certainly continue to ply his resurrected career in the 147lb division.
"It feels good to be a welterweight again. I felt light on my feet," he said.
On the undercard, Vivian Harris of Guyana (22-2-1, 18 KOs) became the number one contender for Junior Witter's WBC light welterweight title with a unanimous decision over Juan Lazcano.
Witter's promoter Mick Hennessy is keen for a fight to happen in the summer.