The 70s heartthrob had been hospitalised last week and died on Tuesday surrounded by family.
His representative said in a statement: “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.
“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 of love and support you have shown him these many years.”
Cassidy, who starred as Keith Partridge in the 1970s TV series The Partridge Family, was said to have been in need of a liver transplant when he was rushed to hospital in Florida.
He also revealed he was battling dementia earlier this year.
The star, who enjoyed hits in the 1970s such as How Can I Be Sure, said he had decided to stop touring, adding: “I want to focus on what I am... I want to love. I want to enjoy life.”
His nephew Jack Cassidy said on Twitter: “My uncle David Cassidy has sadly passed away tonight... & in the process of mourning I can’t help but thank God for the joy that he brought to countless millions of people! I don’t think I’m alone in saying that we will all miss him. God Speed!”
Tributes also poured in on social media from fans and fellow celebrities.
Beach Boys musician Brian Wilson tweeted: “I’m very sad to hear about David Cassidy. There were times in the mid-1970s when he would come over to my house and we even started writing a song together. He was a very talented and nice person. Love & Mercy to David and his family.”
Singer and talk show host Harry Connick Jr said on the social networking site: “So sad to hear of the passing of david cassidy... he was always so kind to me - such a pleasure to have had him on my show... sending love and prayers to his family... R.I.P. friend.”
And television and radio host Larry King tweeted: “Sad to learn David Cassidy has died. Like his father Jack he had great talent, and a complicated life. Condolences to his wonderful family.”
Leading the tributes was Beach Boys founder Brian Wilson, who remembered how he had spent time with Cassidy in the mid-1970s.
“I’m very sad to hear about David Cassidy,” he wrote on Twitter.
“There were times in the mid-1970s when he would come over to my house and we even started writing a song together. He was a very talented and nice person. Love & Mercy to David and his family.”
I Will Survive singer Gloria Gaynor also wrote a poignant message on the social networking site.
She said: “My thoughts and prayers are with the family and loved ones of David Cassidy ... part of a musical legacy via his role as “Keith Partridge” that brought music and laughter into the homes of millions ...”
Musician Marie Osmond, member of showbiz family The Osmonds, shared a photo of old magazines featuring the teen idol.
She added: “Heartbroken over the passing of #DavidCassidy. He graced the covers of teen magazines w/ my Brothers in the ‘70s. My condolences to his Family.”