Baltimore County police say Pierce was initially stopped for speeding around 2AM -- and during the stop, the officer suspected the 24-year-old was hammered.
Pierce was asked to step out of his ride -- and then bombed some field sobriety tests. He was eventually arrested for DUI and hauled to a nearby station where he was processed and later released.
A rep for the Ravens says, "We are aware of the situation and learning more about it. We take these matters seriously."
It's not the first time Pierce has had issues with booze -- TMZ Sports broke the story, Bernard and a couple of his teammates were booted from a Maryland bar during a party last year because "they couldn't maintain their composure."