Cassidy died Tuesday in a Florida hospital at the age of 67 after he was hospitalized in critical condition.
"The Partridge Family" icon and singer was said to be surrounded by family when he died, including his daughter Katie of "Arrow" fame and his son Beau, his brothers Shaun, Patrick and Ryan, his nephews and his ex-wife Sue.
Family sources have said the singer was on life support Monday and his loved ones decided to remove him from it on Tuesday.
"So very sad to hear of David Cassidy passing away," McCormick of "The Brady Brunch" fame said on Twitter.
"He was always so kind and sweet to me. Our shows were both on Friday nights and deep down I dreamt of being a Partridge. His memory and love will live on in my [heart emoji] forever. Love you David. Prayers to his family."
"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," said Wilson of The Beach Boys.
"He was a very talented and nice person. Love & Mercy to David and his family."
"Arrow" lead Stephen Amell, who worked with Katie on the superhero series, also commented on the star's death.
"Arrow is a family & we're all thinking about @MzKatieCassidy & her family tonight," the actor said.
"I grew up in an era before even cable TV, when The Partridge Family was already in reruns. David Cassidy's Keith was one of my favorite TV characters. He was legit funny AND he could sing. The man entertained me during my childhood and even years later," said filmmaker Kevin Smith who shared a video featuring Cassidy and his "Partridge Family" costar Susan Dey.
"I was very sad to read of the loss of David Cassidy. My family and I send much love to his family, friends & many fans," said actor Sean Astin.
"RIP David Cassidy. Who my age did not want to look like, sound like, just be him when they were young?" said actor Tom Arnold. "I was lucky to get to know the sweet & very funny real David as an adult & I still wanted to be him."
"Sad to learn David Cassidy has died. Like his father Jack he had great talent, and a complicated life. Condolences to his wonderful family," said broadcaster Larry King.
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