The former U.S. Open champion, once ranked No. 1, initially planned to quit after the season. Now, with injuries having exacted too much and her thoughts turning to family and a "new life," her retirement is immediate.
"Health and private happiness are so much more important," she said on her Web site Sunday.
Still ranked No. 4, Clijsters said she was wary of the injuries that have led to fatigue, lack of motivation and a decline in play.
"It's tiring to get out of bed and to use an hour just to warm up stiff muscles in the morning," she wrote in Dutch on her online diary. "The constant injuries and continual rehabilitation ... it makes it all even more difficult to go on."
Clijsters made the decision after losing in straight sets to Ukrainian qualifier Julia Vakulenko in her final match Thursday, failing to defend her title in Warsaw, Poland.
Clijsters has been playing for 10 years and won 34 career singles titles, including the 2002 and 2003 WTA Championships. She had wanted to go out with style, making Wimbledon the last Grand Slam tournament of her career. Instead, her summer will be marked by her July 14 wedding to Brian Lynch, a former Villanova basketball player now playing in Belgium.
"It has been more than fun, but the rackets are being hung up," she said. "To retire before the age of 24, it is very young - but it was so beautiful. I would have been able to continue for a few months and to take part in the four most lucrative tournaments (three Grand Slams and the Masters). Money is important, but not the most important thing in my life."
A left wrist injury last year stopped Clijsters from defending her only Gland Slam title in New York and from playing in the Fed Cup final. She reached the 2007 Australian Open semifinals but injured her hip. She recently complained of back pain after losing in Key Biscayne, Fla.
Clijsters was Belgium's first tennis player to reach No. 1, holding the spot for 19 weeks in 2003. But she was unlucky in Grand Slam finals - losing in 2001 and 2003 at the French Open, in 2003 at the U.S. Open and in 2004 at the Australian Open.
Belgian rival Justine Henin praised Clijsters for what she did in her career - "as a player, as a person."
"We've almost grown up together and I think we've helped each other to come in another level because we've pushed each other always to play better. ... She did a lot for the game, for Belgian tennis for sure, and I think I will have great memories of her," Henin said.
Clijsters made an emotional farewell to her Belgian fans in Antwerp at the Diamond Games in February, where she lost the final to Amelie Mauresmo. Antwerp was her last and only tournament on home turf this season.
However, Clijsters promised her fans Sunday that she would organize another goodbye party in September.
"The most beautiful memories of my career?" she wrote on her Web site. "Many victories in tournaments, Grand Slams in singles and doubles as well as being world No. 1 in singles and doubles."
She added: "It is time for a new life. Time for marriage. Time for children? Time also to relax and to play with my dogs. And especially to spend a lot of time with my family and friends."