She entered the plea Wednesday after being charged in an April incident in which she allegedly struck an L.A. County Sheriff's Department vehicle.
The not guilty plea was entered in court by her attorney. The troubled Bynes, 26, was arrested about 3 a.m. on April 6 in West Hollywood, Calif.
Authorities said her black BMW struck the rear corner of the sheriff's car.
Amanda Bynes, Bandage Dress
Bynes was arrested for DUI, but denied the allegations on Twitter: "I can't help but laugh at all of you writing fake stories about me," she tweeted May 27.
"I was not in any hit and runs. I don't drink so the DUI is false."
Tuesday, she tweeted again ... to the Prez: "Hey @BarackObama…I don't drink. Please fire the cop who arrested me. I also don't hit and run. The end."
Bynes is also charged with refusing a breathalyzer or blood-alcohol test at the scene, which could make her sentence worse if she is in fact convicted.
She is scheduled to appear before the same judge who oversaw much of Lindsay Lohan's infamous DUI case ... not a good thing for the actress.
[Photo: WENN.com]
