Manny Pacquiao says he honestly believes UFC champ Ronda Rousey could BEAT UP Floyd Mayweather in an MMA fight ... saying, "She's strong enough."

Manny made the comments on "His & Hers" on ESPN -- saying, "She's strong. She's strong enough to beat Floyd Mayweather in MMA."

And Pacman's not the only one who believes it -- Ronda does too!

In fact, she went on Power 106 last year and said she already has a battle plan if they ever get in the octagon together.