Terence Crawford is undisputed world champion after beating Julius Indongo
American light-welterweight Terence Crawford became the first undisputed men's four-belt world champion in 12 years by knocking out Julius Indongo.
WBC and WBO champion Crawford dropped Indongo with a left hook to the body in round three to claim the WBA and IBF titles in his home state Nebraska.
Middleweight Jermain Taylor was the last man to hold all four major sanctioning organisation belts in 2005.
"It means everything," said Crawford, who is unbeaten in 32 bouts.
"When you start boxing when you're seven years old, that's your dream to become world champion - and after that you want to become something bigger than world champion."
Crawford, 29, had already knocked down Namibia's Indongo in the second round with a right to the body before a powerful left hook to the chest sent the 34-year-old to the canvas as he failed to beat the count.
On the undercard, British heavyweight Dillian Whyte stopped Malcolm Tann in three rounds to win his debut fight in the US.
The 29-year-old dropped American Tann, 39, four times in total to improve his record to 21 wins and one defeat, which was to compatriot Anthony Joshua in 2015.
Crawford could now move up to welterweight, and promoter Bob Arum has said the American may face the winner of the rematch between Manny Pacquiao and Jeff Horn.
"I'm all for it," said Crawford.
Crawford is the first fighter to unify all four belts since Bernard Hopkins knocked out fellow American middleweight world champion Oscar de la Hoya in September 2004 at the age of 39.
Hopkins successfully defended all four titles against Britain's Howard Eastman but then lost them to compatriot Taylor on a split decision in July 2005 - his first defeat in 12 years.
Taylor beat Hopkins again in December 2005 but had to vacate the IBF title in order to take up the rematch, before he was also stripped of the WBA title for not paying sanctioning fees and finally lost the WBC and WBO titles to American Kelly Pavlik in 2007.
After making a comeback in 2011, Taylor eventually regained the IBF title with victory over Australia's Sam Soliman in 2014 but was again stripped of the belt after being arrested for two gun-related incidents in 2015.