The Pasadena Star-News reported Marston Hefner was released from jail Monday after posting $20,000 bail.
Officers were called Sunday night to the apartment Hefner shares with his girlfriend and he was arrested after she was found with minor injuries, Pasadena police said.
Lt. Jari Faulkner said Sinclair, 20, sought an emergency restraining order against Hefner from police
It was unclear if the order was granted.
The Daily Mail reports Sinclair was photographed with a large bruise on her arm while taking some of her belongings out of the Pasadena apartment.
Marston Hefner reportedly kicked Sinclair and punched her in the stomach during a fight the couple had late that night. Police responded to the residence after receiving worried calls from Sinclair's family.
Marston Hefner is one of two adult sons of the Playboy founder and his former wife, ex-playmate Kimberly Conrad Hefner.
On her Twitter account, Sinclair wrote: "There's two types of pain in the world - pain that hurts you, and pain that makes you stronger. All of your positivity is giving me strength."
Sinclair has been dating Marston Hefner since August 2010.