Al Pacino is feeling great at 77, but apparently his hot girlfriend's boob feels even better.

Al was down in Mexico Tuesday celebrating his birthday with gf Lucila Sola ... who's just about 40 years younger than Al. Looks like he's totally tuned in to her.

During a stroll down the beach, Al snuck in a feel -- or maybe he was trying to regain his balance. Hard to say.

Whatever the case ... Al looks great, Lucila does too. Feliz cumpleaños, bud!