Shia LaBeouf -- Showing Off His LaBulge

Shia LaBeouf did his best impression of a teenager on the high school wrestling team Saturday in Studio City ... rolling around town in a killer pair of spandex.

The 28-year-old actor and his girlfriend Mia Goth hit the town for a lunch date and we're guessing he just finished some sort of aerobic exercise. Or maybe he's just incredibly fashion forward in a way us mortals just don't comprehend.

In case the tank top looks familiar ... it appears to be the same one Shia wore during his extremely dramatic ice bucket challenge.

Tight.