Ben Affleck Revealed a BIG Secret Dolphin Back Tattoo!

While it's been reported in the past that he suppressed a high school girlfriend's name with some ink of the sea mammal, we've never seen it before ... until now.

Jen Garner's man -- who's sporting a Justin Bieber 'do for a new movie -- showed off the ink while out with his family over the weekend.

Besides the dolphin, Affleck also has a barb wire tatt on his bicep, a cross on his left arm and roman numerals on his shoulder.