Matthew Perry's loved ones and co-stars say they have been left "heartbroken" by his death aged 54.
The US actor - best known for playing Chandler Bing in hit '90s show Friends - was found dead at his LA home.
The sitcom's creators said he was a "brilliant talent" and "always the funniest person in the room", while his family said he had "brought so much joy to the world".
Perry was praised in recent years for speaking openly about mental health.
Friends, which followed the fortunes of six friends living in New York, aired from 1994 until 2004 and became a cultural touchstone of the era.
More than 52 million viewers in the US alone tuned in for its 2004 finale, making it the most watched TV episode of the 2000s.
The creators of the show, Marta Kauffman and David Crane, and its executive producer Kevin Bright, said Perry's death "still seems impossible", Deadline reported.
In a joint statement they said: "We will always cherish the joy, the light, the blinding intelligence he brought to every moment - not just to his work, but in life as well.
"He was always the funniest person in the room. More than that, he was the sweetest, with a giving and selfless heart."
Referencing the style used for the titles of Friends episodes, they finished their tribute: "This truly is The One Where Our Hearts Are Broken."
In a statement issued to People magazine, his family said they were "heartbroken", adding: "Matthew brought so much joy to the world, both as an actor and a friend."
Perry's fans "meant so much to him and we appreciate the tremendous outpouring of love", they continued.
Actress Gwyneth Paltrow shared how she and Perry spent a "magical summer" together in 1993, where they swam in creeks, shared beers and "kissed in a field of long grass".
"He had shot the pilot of Friends but it had not aired yet. He was nervous, hoping his big break was just around the corner. It was."
She added: "I hope Matthew is at peace at long last. I really do."
Maggie Wheeler, who played on-screen girlfriend Janice, said the joy he brought to so many in his "too short lifetime will live on".
Morgan Fairchild, who played Perry's on-screen mother and erotic novel writer Nora Bing, said: "I'm heartbroken about the untimely death of my 'son', Matthew Perry."
"The loss of such a brilliant young actor is a shock," she added.
Paget Brewster, who played love interest Kathy in the fourth series of Friends, said Perry "won't rest in peace" because "he's already too busy making everyone laugh up there".
In a post on X, formerly known as Twitter, she urged people to read Perry's 2022 memoir Friends, Lovers and the Big Terrible Thing, describing it as his "legacy to help".
The book recounts his struggles with addiction to painkillers and alcohol during his sudden rise to fame, and lifelong struggles with mental health problems.
Perry experienced health problems including a burst colon in 2018 due to his drug usage, which required several operations.
In 2016, he told BBC Radio 2 that he could not remember three years of filming during Friends, because of his drink and drug use.
Perry's efforts to help others struggling with addiction was reflected in a tribute from actor Hank Azaria, who appeared in five episodes of Friends.
Azaria said Perry had been a "brother" when the pair first moved to LA, and revealed it was Perry who took him to his first Alcoholics Anonymous meeting when he was struggling with addiction.
Born in Massachusetts in 1969, Perry was raised in Ottawa, Canada, where he attended elementary school with Justin Trudeau, who later became Canadian prime minister.
Canada's leader wrote on X: "Matthew Perry's passing is shocking and saddening. I'll never forget the schoolyard games we used to play, and I know people around the world are never going to forget the joy he brought them.
"Thanks for all the laughs, Matthew. You were loved - and you will be missed."
Perry moved to Los Angeles as a teenager. He starred as Chazz Russell in Boys Will Be Boys and had roles in shows including Growing Pains.
But it was his role in Friends - alongside Jennifer Aniston, Courteney Cox, David Schwimmer, Matt LeBlanc and Lisa Kudrow - that launched him to international stardom.
Perry was cast as the awkward Chandler Bing, famous for his sarcastic one-liners and his childish behaviour with room-mate Joey Tribbiani.
The role earned him an Emmy nomination in 2002. Regular reruns and streaming services have allowed it to find a new generation of devotees.
Tributes have also been paid by famous fans of the show, including singer Adele, who said she felt she had a connection with Perry after decades of watching Friends, despite having never met him in person.
"I just want to say how much I love what he did for all of us," she said in a video message.
Warner Bros, which produced the show, described Perry as a "true gift to us all".
Despite the universal popularity of Friends, Perry spoke about not being able to watch the show in an in an interview last year.
"I didn't watch the show, and haven't watched the show, because I could go, 'Drinking, opiates, drinking, cocaine,'" he said. "I could tell season by season, by how I looked. That's why I don't wanna watch it, because that's what I see."
Perry's film credits include fools Rush In, Almost Heroes and the Whole Nine Yards and he won critical acclaim for later TV roles.
His role as associate White House counsel Joe Quincy in Aaron Sorkin's The West Wing earned him two Emmy nominations for outstanding guest actor in a drama series in 2003 and 2004.
He played the lead role in Sorkin's later series, Studio 60 On The Sunset Strip, then wrote and produced his own show called Mr Sunshine in which he also played the lead character.
Lucy Davis, who starred in Studio 60 on the Sunset Strip, said it was an "honour" to work with Perry on the show.
In a post on X, she wrote: "He was as funny off screen as he was on screen. He was really kind to me: I had one week where I was having difficulty with something at work and he came up to me and told me not to worry because he had my back.
"Thanks for all the laughs Matthew. You made a great impact on me."
The Friends cast reunited in 2021 for a televised reunion, where Perry once again opened up about his mental health struggles during the peak of the show's success.
He recalled "freaking out" and feeling "like I would die" if the audience didn't laugh at one of his lines, describing his mindset at the time as "not healthy".
The LA Times and TMZ, which first reported that Perry had died, said the actor was found unresponsive in a hot tub at his house.
A Los Angeles Fire Department spokesperson told the BBC that first responders went to an address in the Pacific Palisades area, regarding a "water emergency" of an unknown type, but did not name Perry.
Perry's final Instagram post, published a few days ago, shows him relaxing in a rooftop pool. It has since attracted a stream of tributes from fans.
Perry leaves parents John Bennett Perry and Suzanne Perry, who split up when he was a baby. He has five half-siblings. He never married and had no children.