The 60-year-old former teen idol entered the plea at a hearing on Wednesday, but he did not have to appear in court since the charge was a misdemeanor, and he had no prior record.
Cassidy was arrested after failing a field sobriety test on Nov. 3. Officers said they stopped Cassidy after seeing his Mercedes swerve on State Road 91 just after 6 p.m. They said Cassidy was weaving across lanes and cut off another car, nearly causing an accident.
After Cassidy was stopped, officers said they smelled alcohol when he was talking to them. He told officers that he had a glass of wine at lunch and a hydrocodone tablet later in the afternoon.
When he got out of his car, officers said Cassidy was unsteady on his feet and failed several field sobriety tests. Breath tests at the St. Lucie County jail showed his blood-alcohol content at 0.139 and 0.141, above Florida's legal limit of 0.08.
Officers searched his car and said they found a bottle of bourbon that was half-empty in the back, according to an arrest report.
The Associated Press contributed to this story.