Hugh Hefner's 21-year-old son Marston was arrested Sunday night for attacking his girlfriend.

As you might expect, the son of Hugh Hefner doesn't just date random chicks.  Marston's girlfriend is 2011 Playmate of the Year Claire Sinclair.

She told police Marston punched her and kicked her in the stomach.  Police observed bruises and red marks on her body.  Marston was booked for misdemeanor domestic violence and released on $20,000 bail.

Claire went to court yesterday to get a temporary restraining order against Marston so she can move her things out of the house they were sharing.

She also wants Marston to give her a public apology AND get psychiatric help.  And she says that if he does that, she won't pursue charges.