Chidi Benjamin Uzomah Jr. entered the plea Tuesday in Los Angeles and was ordered to undergo a psychiatric evaluation.
Uzomah was arrested Oct. 30 after he showed up at the E! Entertainment Television headquarters trying to contact Seacrest.
Photos: "American Idol" Season Eight
Photos: Ellen DeGeneres
The incident was a violation of a restraining order issued in another case in which Uzomah admitted attacking one of Seacrest's bodyguards at an event.
The popular host of "American Idol" and a radio show obtained a civil restraining order against the 25-year-old.
The Los Angeles District Attorney's office says Uzomah will be sentenced on June 14. He faces up to two years in state prison.