Mayweather, who's often called the greatest boxer ever, chose Pacquiao over other big names like Canelo Alvarez and Oscar De La Hoya. He wrapped up his pro career in 2017 with a perfect 50-0 record after 21 years in the ring.
The 48-year-old from Grand Rapids, Michigan, explained why he rates Pacquiao so highly:
"The best fighter I fought probably was Manny Pacquiao because of his movement. He's a hell of a fighter and I can see why he won so many fights and I can see why he's going down as a Hall of Famer, there's just certain moves that he makes."
Mayweather and Pacquiao finally met in the ring in 2015 in Las Vegas. The fight was billed as 'The Fight Of The Century', but it didn't quite live up to the hype. Both fighters were past their best years, which left many fans feeling let down. Mayweather won by unanimous decision.
There was talk about a rematch, but it never happened. Mayweather hung up his gloves for good in 2017 after beating UFC star Conor McGregor.
Pacquiao, known as 'PacMan', retired in 2019 after losing to Yordenis Ugas. But he's not done yet. He's planning a comeback and will face Mario Barrios for the WBC welterweight title.
Mayweather's career was full of big wins, but it seems Pacquiao left the biggest mark on him as a fighter.