Among the other terms of the plea, the former star of the TV series "The Partridge Family" must attend DUI school, pay a $500 fine and serve 50 hours of community service, said Gayle Braun, supervisor of the misdemeanor and juvenile divisions of the state attorney's office for the judicial circuit encompassing St. Lucie County.
The attorney for the 60-year-old actor and musician entered the plea for his client, who was not present in court.
A Florida Highway Patrol officer stopped him in Fort Pierce last November after his Mercedes left the roadway and went onto the shoulder rumble strip two times before cutting off another car at an exit ramp, the police report said.
Cassidy failed roadside sobriety tests and registered at 0.14% blood alcohol content on a breath test, nearly twice the state's 0.08% legal limit, according to the arrest report.
The singer-actor "appeared to be somewhat confused" and was "very apologetic for having to be stopped," the arresting officer said.
Police found a half-empty bottle of bourbon under the seat of Cassidy's Mercedes, although he told the officer he had consumed only one glass of wine with lunch several hours earlier, the report said.
He also told the officer he had taken a hydrocodone pill -- a pain medication also known as Vicodin -- about three hours earlier.
On the TV series, Cassidy played the eldest of five children of a widowed mother (Shirley Jones, who was Cassidy's real-life step-mother). The family -- all of them musicians -- traveled to gigs in a repainted school bus. When the show was canceled after four seasons, Cassidy launched his own pop music career, filling concert halls with screaming teen-aged girls attracted to such hits as "I Think I Love You."
His most recent acting job was on the 2009 ABC Family sitcom "Ruby & the Rockits," in which Cassidy played an aging former teen heartthrob. The show was canceled after one season.