Jeremy Piven Accused of Sexual Assault on 'Entourage' Set

Jeremy Piven is being called out by a former "Entourage" extra who claims he groped her on the set of the show and also at the Playboy Mansion.

Ariane Bellamar tweeted Piven Monday, saying ... "Member when you cornered me in your trailer on the #Entourage set? 'Member grabbing my boobies on the [couch] without asking??"

She also claims she tried to leave but Piven "grabbed me by the ass, looked at yourself in the mirror, & said what a 'beautiful couple' we made?" Bellamar, an actress and ex-reality tv star, claims he did the same -- fondling her breasts and butt -- at the Mansion.

We reached out to Piven's camp, but haven't heard back.