The Florida Highway Patrol said the 60-year-old "Partridge Family" actor was pulled over around 6 p.m. Wednesday for weaving across lanes and cutting off another car, nearly causing an accident.
Cassidy told the trooper he'd had a glass of wine at lunch and a hydrocodone pill later in the afternoon. Troopers said they found a half-empty bottle of bourbon in the backseat.
The FHP said Cassidy failed a field sobriety test, and 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.
Cassidy posted bail and was released from jail early Thursday morning.
The Associated Press contributed to this report.