The actor's 20-year-old son Nathen posted a heartbreaking tribute on Instagram, writing: "I love you so much papa.
"Rest In Peace and I’m so glad you got to share all your adventures with me and were able to accomplish so much. I’m so proud of you."
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Willie adopted Nathen in 2009.
Alongside a series of pictures of his father on adventures around the world, he added: "I will always love you, but I think it’s time for you to go on an adventure of your own.
"You’ll always be with me.
"Love you more than you will ever know and I’m glad you can be at peace now. You always were the toughest and funniest and smartest person I’ve known. I’m glad you shared your love with me. I’ll never forget it or lose it."
Willie has died age 57
The news was also shared by his close friend Rob Morrow, who tweeted: "Our sweet pal #WillieGarson has passed on. Dear soul, rest easy."
Mario Cantone, who starred alongside Willie in SATC added: "I couldn’t have had a more brilliant TV partner. I’m devastated and just overwhelmed with Sadness. Taken away from all of us way soon. You were a gift from the gods. Rest my sweet friend. I love you."
In 2020, Willie and Nathen opened up on the adoption, with Willie discussing their love of travel "as long as we stop for ice cream every two hours".
His son Nathen confirmed the tragic news
"Nathen and I always talk," he told People.
"We talk about everything that a lot of his friends’ parents say, ‘I can’t talk to my kid about that.’ We tend to work it out together because we are partners in this."
Added Nathen: "I used to call him Willie, now I call him Dad."
Willie served as a spokesperson for National Adoption Day twice.
Willie (center) filming in July
Born in 1964, he is also known for playing Mozzie in the USA Network series White Collar, and Gerald Hirsch in the reboot of Hawaii Five-0.
He was set to appear in the Sex And The City spin-off, And Just Like That, which is filming in New York City.
He was pictured on set in late July with Sarah Jessica Parker, David Eigenberg and Cynthia Nixon.
He also lent voice to Netflix animation Big Mouth, and had a recurring role on The CW's Supergirl from 2019 to 2020. His cause of death has not yet been released.
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