Sources tell us the Golden Boy booked the White Horse in Pasadena last weekend for Anna Valencia's 30th ... decking the place out in a casino theme, complete with blackjack tables and chips featuring her face!!

We're told the boxing champ footed the entire bill for the night (even the open bar) -- which he can definitely afford after putting together the largest contract in the history of pro sports.

Remember, it was Golden Boy that negotiated Canelo Alvarez's $365 MILLION fight contract with DAZN ... in which Canelo will take 11 fights over 5 years. Oscar's cut has gotta be pretty sweet.

As for Valencia, she's a model and an actress ... and has even done work with Golden Boy Promotions as an on-camera reporter.

The couple has been dating since earlier this year ... and by the looks of it, they couldn't be happier.

Happy birthday!!