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Floyd Mayweather says he'll be right in the middle of the action for Donald Trump's inauguration in D.C. next week ... telling TMZ Sports he's already got his tuxedo picked out!

Turns out, Mayweather's been friends with Trump for a while -- and says he really appreciated the support he got from Donald during the Manny Pacquiao fight.

Mayweather wouldn't say if he's a democrat or a republican -- but praised both Obama and Trump.

"Barack Obama was a good president and hopefully, Donald Trump is a good president."