Ritter became ill Thursday while working on his ABC series and underwent surgery at Providence St. Joseph Medical Center in Burbank for a tear in his aorta, a rare medical condition that can hit without warning.
He died shortly after 10 p.m. Thursday, publicist Lisa Kasteler said. He was accompanied by producers, co-workers, his wife and his 23-year-old son, Jason, said Susan Wilcox, his assistant of 22 years.
Ritter's youngest child, Stella, turned 5 the day he died. His 55th birthday was next Wednesday.
The son of Tex Ritter, a Western film star and country musician, Ritter was an effortless funnyman who -- given the chance -- could handle drama as well. Friends recalled him as loving and buoyant.
"It's like there is a big tear in the world's heart," actor Henry Winkler told "Entertainment Tonight" on Friday. "He was extraordinary in every aspect of his life, especially as a father. His children were there at every moment of his life."
Winkler, who co-starred with Ritter on Broadway in Neil Simon's "The Dinner Party," was to make a guest appearance on the ABC sitcom. He was on the set Thursday for rehearsal when he was told Ritter had taken ill.
No decision had been made Friday about the future of "8 Simple Rules...," which was to begin its second season Sept. 23, an ABC spokesman said. It's one of the few bright spots in the struggling network's lineup. Three new episodes had already been filmed, and Ritter was working on the fourth when he fell ill.
"All of us at ABC, Touchstone Television and The Walt Disney Co. are shocked and heartbroken at the terrible news of John's passing," a statement read. "Our thoughts and prayers are with his wife and children at this very difficult time."
"He was more than a comic," Simon said in a statement. "He was a real actor with a genius for comedy. I loved his performance in 'The Dinner Party.'"
"I'm shocked and heartbroken and so sad for his family. I cannot find words to express my sorrow -- such a great loss to the joy in the world," Joyce DeWitt, who co-starred with Ritter and Suzanne Somers in ABC's "Three's Company," told "Entertainment Tonight."
The sitcom, which aired from 1977-84 and brought a new level of risque humor to TV, was the No. 1 comedy in the 1979-80 season and regularly part of the top 10.
Ritter played a handsome but goofy bachelor who hinted he was gay so he could live with his two female roommates without raising eyebrows. Sexual double-entendres were the order of the day.
Behind the scenes, Somers' money demands led to clashes with Ritter and DeWitt, and she was eventually written off the show. Somers she had reconciled with Ritter at the request of his wife, actress Amy Yasbeck.
"If we had not, today would be unbearable for me," Somers said in a statement. "I am glad I knew him. I am privileged to have worked with him. I am unbelievably sad for his family, and I will miss him."
"It's just stunning, unbelievable," said Wilcox, his assistant. "Everybody loved John Ritter. Everybody loved working with him. ... Whatever set he was working on, he made it a very fun place."
Ritter, a Southern California native who lived in Beverly Hills, had appeared in more than 25 television movies and a number of films.
The youngest son of Tex Ritter and actress Dorothy Fay, he graduated from Hollywood High School and earned a degree in drama from the University of Southern California.
"I was the class clown, but I was also student body president in high school," he told The Associated Press in a 1992 interview. "I had my serious side -- I idolized Bobby Kennedy, he was my role model. But so was Jerry Lewis."