Mark Wahlberg -- $250k Bet w/ Diddy Is REAL ... Money's In Escrow

Anyone who thought that Diddy and Mark Wahlberg's Mayweather vs. Pacquiao mega-bet was just for show is WRONG ... 'cause Mark tells TMZ Sports the $250,000 is already in escrow.

In case you forgot ... the two superstars made a massive bet on the outcome of the fight, with Diddy backing Mayweather and Wahlberg siding with Pacquiao.

Mark was out in Beverly Hills Tuesday when we asked the movie star if he was at all worried about Diddy pulling an Aaron Rodgers ... and backing out of the agreement if Pacman turns out to be victorious.

Check out the clip -- Mark says he's not worried about a Rodgering ... 'cause he CAN'T, due to the fact that the money is already being held by a neutral party.

FYI -- Mark also says the winnings will go to the winning fighter's favorite charity.

Which means that the strippers at Sam's Hofbrau are rooting extra hard for Floyd.