Gotta love a dude who's betting on himself ... after signing the largest contract in NFL history!

Check out Patrick Mahomes, Travis Kelce and their significant others hitting the casino in Lake Tahoe on Thursday night ... keeping the party going after a day on the water.

Unclear if Patrick actually did any wagering -- but it's gotta be a GREAT feeling knowing your bank account will soon be bigger than all of the progressive jackpots in the building COMBINED!