The actor and singer had been admitted recently to a Fort Lauderdale hospital in critical condition, before succumbing to organ failure. Cassidy had battled alcoholism and also suffered dementia.
"On behalf of the entire Cassidy family, it is with great sadness that we announce the passing of our father, our uncle, and our dear brother, David Cassidy," his publicist JoAnn Geffen said in a statement. "David died surrounded by those he loved, with joy in his heart and free from the pain that had gripped him for so long. Thank you for the abundance and support you have shown him these many years."
Cassidy was born into an entertainment family — his father was singer/actor Jack Cassidy and his mother was actress Evelyn Ward. Later, his father married the actress Shirley Jones, who played his mother on "The Partridge Family."
On the musical sitcom, Cassidy played Keith Partridge, the siblings' lead vocalist. The single, "I Think I Love You," hit No. 1 on the Billboard chart in 1970. The group was nominated for Best New Artist at the Grammys.
Cassidy then launched a solo career and recorded dozens of albums. He performed to massive crowds in arenas, but also continued acting in television and in musical theater.
He was married three times to actress Kay Lenz, horse breeder Meryl Tanz, and Sue Shifrin. He had two children, Katie (an actress on "Arrow") and Beau.