Hugh Hefner's Son -- Charged with Beating Playmate GF Claire Sinclair

Hugh Hefner's son Marston Hefner has officially been charged with roughing up his Playmate girlfriend ... and now faces over a year in jail -- this according to the Pasadena City Attorney.

TMZ broke the story ... Marston was arrested earlier this month after allegedly attacking Playmate of the Year Claire Sinclair inside the couple's Pasadena apartment.

A rep for the Pasadena City Attorney's office tells us, Marston was charged yesterday with one count of inflicting corporal injury on a spouse or cohabitant.

We're told Marston has also been charged with vandalism -- after he allegedly damaged Claire's lap top during the heated argument.

If convicted on both counts, Marston faces a year-and-a-half behind bars.