Ritter, who would have been 55 next Wednesday, became ill Thursday on the set of his TV comedy "8 Simple Rules for Dating My Teenage Daughter." He died shortly after 10 p.m. at Providence St. Joseph Medical Center in Burbank, Calif..
His publicist, Lisa Kasteler, said the cause of death was a dissection of the aorta, which resulted from a flaw in his heart that had gone undiagnosed.
Ritter, who was born into a show business family and grew up in Hollywood, became a star in 1977 when he co-starred with Joyce DeWitt and Suzanne Somers in "Three's Company." Considered daring at the time, the half-hour comedy featured lots of double entendre sexual humor and became one of TV's biggest hits during its seven-year run on ABC.
Ritter scored as the awkward but genial Jack Tripper, who shared a Southern California apartment with Janet (DeWitt) and Chrissy (Somers). Although the character was a wolf, he often hinted to the apartment's landlords that he was homosexual -- so as to reassure them that he and his roommates were not engaging in any "hanky panky."
The role earned Ritter Emmy nominations in 1978, 1981 and 1984 for Outstanding Lead Actor in a Comedy Series. He won the Emmy in 1984.
Ritter was also nominated for best actor in a comedy series in 1988 for "Hooperman," in which he starred as a mild-mannered San Francisco police detective. He was nominated for Outstanding Guest Actor in a Comedy Series for his performance in a 1999 episode of "Ally McBeal." He also received two Emmy nominations for his work as the voice of "Clifford the Big Red Dog" on the PBS animated series.
In "8 Simple Rules," Ritter played Paul Hennessy -- a sportswriter who has spent years on the road but who decided to take on more parental duties when his wife (Katey Sagal) decided to pursue a career in nursing. The show was scheduled to begin its second season Sept. 23.
ABC issued a statement expressing shock and sorrow at Ritter's death.
"All of us at ABC, Touchstone Television and The Walt Disney Company are shocked and heartbroken at the terrible news of John's passing," said the statement. "Our thoughts and prayers are with his wife and children at this very difficult time."
Ritter was married to actress Amy Yasbeck, with whom he had a daughter, Stella, 5. He had three children -- Jason, Tyler and Carly -- with his first wife, actress Nancy Morgan. They were divorced in 1996.
Jason Ritter has followed in the family business. He recently appeared in the box-office hit "Freddy Vs. Jason." His other movie credits include "Swimfan" (2002) and "Mumford" (1999).
John Ritter was born on Sept. 17, 1948, in Burbank. His father was the singing cowboy star Tex Ritter. His mother, Dorothy Fay Ritter, was an official greeter at The Grand Ole Opry in Nashville.
Ritter served as student body president at Hollywood High School, and went on to earn a degree in drama from the University of Southern California in 1971 -- the same year he made his feature film debut in the Disney comedy "The Barefoot Executive."
He made dozens of guest-star appearances in TV series -- including "The Love Boat," "Rhoda," "Starsky and Hutch," "The Mary Tyler Moore Show," "The Bob Newhart Show," "Kojak" and "M*A*S*H." He also appeared frequently on "The Waltons" as the Rev. Matthew Fosdick.
"Three's Company" changed everything, and Ritter became one of the biggest stars in TV. Viewers fell in love with his character's goofy good nature, and his consistent ability to bounce back whenever he fell -- which was just about every episode.
In a 2002 interview, Ritter told the Washington Post it was nice to have a hit TV series again, and he had high hopes for a long run for the show.
"People can use a laugh these days and the situations may be familiar -- or viewers may be glad that they don't have these problems," he said. "The writing is so good ... I'd like to see (several) seasons, then start 'Simple Rules for Dating My Granddaughters.'"
Although "8 Simple Rules" is often referred to as a "comeback" for Ritter, the truth is he never even slowed down after "Three's Company." In addition to "Hooperman," Ritter co-starred with Markie Post from 1992-95 in the politics-themed CBS comedy series "Hearts Afire." The series featured Billy Bob Thornton, who later cast Ritter in his Oscar-winning 1996 drama "Sling Blade."
Ritter's TV movie appearances included Stephen King's "It" and "Unnatural Causes." In recent years, along with "Ally McBeal," Ritter had guest-starred in such TV hits at "Buffy the Vampire Slayer," "Chicago Hope" and "Touched by an Angel."
His feature credits included "Problem Child," "Bride of Chucky" and "Noises Off."
He has two movies scheduled for release this year. In "Bad Santa," he once again co-stars with Thornton. "Manhood," a black comedy, pairs him with Janeane Garofalo and Nestor Carbonell.
In 2000, Ritter starred on Broadway in Neil Simon's "The Dinner Party." The Los Angeles Music Center honored Ritter last June with its Distinguished Artist Award.
Ritter hosted several national telethons to help raise funds for United Cerebral Palsy with his older brother Tom, a lawyer, who has CP. Earlier this year, Ritter was part of an all-star cast for a staged reading of "All About Eve" in Los Angeles, to raise funds for the Actors Fund of America. The cast included Stockard Channing, Kirk Douglas, Angela Lansbury and Carl Reiner.