The former pop idol is in a "critical condition" in hospital after reportedly suffering from organ failure.
A US report has suggested the former pop idol has been admitted to a medical centre in Florida and placed in an induced coma.
TMZ.com reports Cassidy, 67, is in need of a liver transplant and is suffering from kidney failure.
He is said to have been taken to hospital three days ago after suffering months of deteriorating health.
The website suggests his condition worsened over the last few days and he is now listed as "critical".
His family members are said to be by his side at the hospital.
The report by TMZ suggests The Partridge Family star has been placed in an induced coma, but a representative for the singer/actor has insisted the veteran star is awake.
They told The Mirror: "He is currently conscious surrounded by family."
He has reportedly been suffering health problems for two months.
Earlier this year, Cassidy revealed he is battling dementia and planned to stop touring to concentrate on his health.
He confirmed he was fighting the disease after he was filmed slurring and falling off stage during a gig.
The veteran star, who shot to fame as Keith in TV sitcom The Partridge Family told People magazine: “I was in denial, but a part of me always knew it was coming.
“I want to focus on what I am, who I am and how I’ve been without any distractions. I want to love. I want to enjoy life.”
Cassidy, who has suffered from alcohol problems in the past, also said he had decided to stop touring after struggling to remember lyrics to his classic songs.
As well as health issues, Cassidy also suffered a string of legal problems in recent years, including several arrests on suspicion of drink-driving, and a court-ordered rehab stint in 2014.
The actor, who received an Emmy nomination for his appearance on Police Story in 1978, was most recently seen on TV in a 2013 episode of CSI: Crime Scene Investigation.