The Filipino, in his 70th fight and his first as a 40-year-old, rocked the former four-weight world champion with a stunning left in the ninth round, BBC Sport reported.
He dominated his American opponent, who is 11 years his junior, connecting with more than twice as many punches.
Judges scored the bout 117-111, 116-112, 116-112 in Pacquiao's favor.
Afterward, Pacquiao said he would be open to a rematch with unbeaten American Floyd Mayweather, who defeated him in 2015.
"Tell him to come back to the ring and we will fight," said Pacquiao, who is a senator in his homeland and fighting in the US for the first time in two years.
"I am willing to fight with Floyd Mayweather if he is willing to come back to box."
Former five-weight world champion Mayweather, who was in the crowd, gave no reaction when asked if the rematch could happen.
The 41-year-old beat Japanese kickboxer Tenshin Nasukawa in an exhibition boxing bout in December but has previously said he is "still retired".
Pacquaio, securing the 61st victory of his career, almost knocked down his opponent in the seventh and ninth rounds.
But afterward, Broner – to jeers from the MGM Grand crowd – claimed he should have won.
"I beat him. Everyone out there knows I beat him. I controlled the fight, he was missing. I hit clean more times. I beat him," he said.
Broner had 50 connecting punches to Pacquaio's 112 and three connecting body shots to the victor's 47.
Earlier, American Marcus Browne took the vacant WBA interim light-heavyweight title with a unanimous points win over Sweden's Badou Jack.
Jack suffered a nasty cut to the middle of his forehead but managed to complete the final five rounds before losing on points by 117-110, 116-111, 119-108.
French fighter Nordine Oubaali earned his first world title by beating American Rau'shee Warren by unanimous points decision to take the WBC bantamweight belt.