Porn Star Mr. Marcus -- Putting the Big D ... In DUI

Porn stud Mr. Marcus -- who, we kid you not, is a member of the porn Hall of Fame -- was arrested in Hollywood early this morning, after cops say he was driving drunk ... TMZ has learned.

Law enforcement tells us ... Mr. Marcus -- star of "Phat Ass White Booty 6" and "Manhammer 7" -- was pulled over in a Hyundai Elantra around 2:30 AM after cops noticed the car weaving all over the road.

During the stop, we're told cops smelled booze, so they performed a field sobriety test.

Sources tell us ... Marcus bombed the breathalyzer (blowing isn't really his forte) ... so he was arrested for DUI.