Josh Allen appears to be getting a lot more serious with Hailee Steinfeld … the two were spotted in a heavy makeout sesh while on vacation in Mexico!!!

The Buffalo Bills quarterback and the actress spent their Fourth of July south of the border, packing on the PDA during a romantic getaway.

Josh and Hailee looked to be all over each other in the resort pool, wrapping their arms around one another and sucking face.

Hailee showed off her body with a red bikini ... and Josh showed off his muscles with some board shorts ... no wonder they couldn't keep their hands -- and lips -- to themselves.

Josh and Hailee haven't publicly confirmed they are in a romantic relationship yet ... but the signs clearly appear to be there ... and we previously saw them get cozy back in May during a sushi dinner in New York City.

Now, Josh and Hailee are going international ... and kissing.