A statement from his family said: "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."
David first came to the world's attention as the star of the smash hit TV show The Partridge Family, which aired from 1970-74.
The show gave him an audience for his music and his first major single 'I think I Love You' hit number one, selling five million copies.
At one point, his fan club had more members than Elvis or The Beatles.
His clean-cut image changed when he gave an interview to Rolling Stones magazine, posing naked on the cover and confessing to experimenting with drugs.
Cassidy hit the headlines in later life when he was arrested on numerous occasions for drink-driving charges. He bravely revealed that he was suffering from alcoholism.
But his greatest battle was to follow earlier this year when he was diagnosed with dementia.

The star's nephew Jack Cassidy, a singer himself, took to Twitter to pay his respect. "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!" he wrote

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!

— Jack Cassidy (@jackgcassidy) November 22, 2017