A rep for the former Partridge Family star announced the sad news in a statement Tuesday night.
Cassidy entered a hospital over the weekend for treatment of liver failure.
Earlier this year, Cassidy revealed he was battling dementia. The popular show catapulted Cassidy to heartthrob status, and he toured the world to sing hits from it such as "I Think I Love You" before throngs of teen girls.
Cassidy acted in television and film through the years, appearing last in a "CSI: Crime Scene Investigation" episode in 2013. His publicist said Cassidy was listed in critical condition at a hospital in Fort Lauderdale, Florida.
Cassidy's stepmother, Shirley Jones, was married to Jack for almost two decades until they divorced in 1975.
By the 1980s Cassidy was broke, but he continued recording and performing, starring on Broadway in Joseph and the Amazing Technicolor Dreamcoat and later in Blood Brothers with half-brother Shaun Cassidy. He also made guest appearances on shows including "Fantasy Island", "The Love Boat" and "Tales of the Unexpected". The songs "Doesn't Somebody Want To Be Wanted" and "I'll Meet You Halfway" also cracked the top 10 the next year.
Cassidy fared better in Las Vegas.
"When friends of yours or family members begin to say to you, 'Remember, I just told you this two days ago, ' and there's no memory of it, that's when I began to be very concerned", he said on the show.
Cassidy hit some low points and ended up in rehab in 2014 after his third arrest for driving under the influence.
"The Partridge Family" aired from 1970-74 and was intended at first as a vehicle for Shirley Jones, the Oscar winning actress and Cassidy's stepmother.
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