Matthew McConaughey was barely recognizable Monday on the set of his new flick, "The Beach Bum" ... although the role doesn't look like much of a stretch.

Matt's down in Miami shooting the new flick where he plays Moondog ... a dude who "lives life by his own rules." We smell some serious method acting. Isla Fisher is also in it, but her look's more apocalyptic than beach-y.

Saying McConaughey was born to play this role would be an understatement.

Remember, the man's already got his Oscar ... he can take whatever role he wants.