Donaire improved to 29-1 with 18 knockouts, winning his 28th straight fight
Nonito Donaire added Jeffrey Mathebula's International Boxing Federation super bantamweight title to his own World Boxing Organization belt with a 12-round unanimous decision.
Filipino star Donaire improved to 29-1 with 18 knockouts, winning his 28th straight fight in a streak dating back to 2001.
He retained the WBO crown he won in February in a split decision over Puerto Rico's Wilfredo Vazquez.
Donaire was the aggressor against his much taller opponent and dropped South African Mathebula with a devastating left late in the fourth round. It was the first time in his career that the 33-year-old Mathebula had been sent down.
"He didn't expect it. Once he got hit with that, he was like everyone else," Donaire said. "He kept his hands up the rest of the night."
Mathebula, making good use of his height and reach advantage, stayed active, his work rate keeping him in the contest even as Donaire landed the harder blows.
In the 11th, Donaire landed a stinging right to Mathebula's jaw that cracked one of the South African's teeth.
The ringside judges scored it 117-110, 118-109 and 119-108 in favor of Donaire.
"He was a tough guy, I didn't expect him to be... He was faster than I thought," said Donaire. "It was difficult to counter him with the right. If it would have been easy to counter him, it would've been an easy fight.
"He got me off that jab really well," Donaire added. "He's a great champion."
Mathebula fell to 26-4 with two drawn and 14 knockouts. He was making the first defence of the IBF title he won in March in South Africa with a split-decision triumph over compatriot Takalani Ndlovu.
The bout was the first step in what Donaire hopes will be a unification march through the division.
The World Boxing Association super bantamweight belt is held by Guillermo Rigondeaux, a 31-year-old Cuban who is 10-0 as a professional and won Olympic gold in 2000 and 2004.
Mexico's Abner Mares owns the World Boxing Council super bantamweight crown but Donaire has voiced an interest in taking on the man Mares replaced as champion, Japan's Toshiaki Nishioka.
Nishioka, a 35-year-old southpaw, is 39-4 with three drawn and has won 16 fights in a row since last losing in 2004 to Thai legend Veeraphol Sahaprom.
Donaire said he would like to collect one more belt, then move up to 126 pounds.
"We have a lot of guys, like Nishioka, who's a great fighter," Donaire said. "We have Mares, Rigondeaux. One more belt and I'm good to go to 126."