City Attorney's spokesman Frank Mateljan said Petty entered the plea through her attorney and has already completed some of the terms of her sentence, which include alcohol abuse counseling.
Petty was arrested May 30 after a skateboarder was hit with a car in the Venice Beach area, authorities said.
"I sincerely apologize for driving after drinking alcohol," Petty said in a written statement.
She urged people not to drive after drinking and to take keys from friends if they appear to be under the influence.
"I assure you, this first misdemeanor DUI will be my last," her statement read.
Petty's film credits include "Point Break," "Tank Girl" and "A League of Their Own." The 45-year-old actress was not required to appear in court