District Attorney's spokeswoman Jane Robison says a public defender entered the plea on 24-year-old Zachary Loring's behalf.
Video shot by celebrity Web site TMZ.com shows Loring trying to rescind the plea and making several obscene gestures before a court commissioner ordered taping to stop.
Loring was arrested Friday outside the home of Patridge, who is 1 of the stars of MTV's "The Hills." He is accused of violating a restraining order the reality TV star obtained against Loring last year.
Robison says Loring's bail was set at $150,000 and another protective order was issued.
