Ya gotta see these pics of the actors enjoying a day party on the famous Greek island of Mykonos ... Matt's got a cold beer in his hand and looks super animated, definitely having fun.

Matt, Chris and Liam posed for a ton of pics with their group taking snaps on smartphones ... mugging for the cameras and even flipping the bird at times. Yeah, it's a party.

It's not just the Hollywood hunk on the trip, either ... Matt's dancing with his wife Luciana Barroso, Liam's traveling with his model girlfriend, Gabriella Brooks, and there's some other ladies in the group.

Matt and Chris go way back ... they've been close friends for over a decade and have worked on a handful of movies together during their careers.