Armstrong, 33, will attempt to win his record seventh straight Tour de France in July. His streak of six straight titles has made him a legend in the sport and an international celebrity.
"I have decided the Tour de France will be my last race as a professional cyclist," he said.
"It will be the last one, win or lose."
Armstrong said he began thinking about retirement after winning his sixth Tour de France last year. He said having to spend one month away from his children recently helped to complete the decision.
"That was much more difficult that it had been before," he said. "They are at a stage now where they change daily, if not hourly. ... It's time for me to not miss key moments in their lives."
Armstrong said he remains "fully committed" to winning this year's Tour de France and is driven "by that dream to go out on top. That's a big deal to me."
Armstrong confirmed retirement plans that had become the focus of growing speculation as he spoke in recent months that he wants to spend more time with his three children and in his campaign against cancer.
"Ultimately, athletes have to retire ... the body doesn't just keep going and going," Armstrong said.
He acknowledged that it will not be easy to stick with his retirement decision in one year, when other racers are gearing up for the 2006 Tour de France. He said that while watching a recent cycling race on TV with his girlfriend, rock and roll star Sheryl Crow (search), he was so stirred by the competition he said "I couldn't sit down the entire race."
Armstrong said at that point Crow challenged him on his retirement decision.
"She said 'Look at you. You can't even sit down. How are you going to retire?' " Armstrong said.
"It's a great question. I have to tell you I am 100 percent committed and the decision is final. ... I'll definitely have the itch now and again."
Monday's announcement came on the eve of Armstrong's defense of his Tour de Georgia championship. The six-day, 648-mile event he uses as a training tool for the Tour de France begins Tuesday in Augusta.
Armstrong said the Georgia race could be his last professional competition in the U.S., though he left open the possibility of finding a local race in May as another practice event before the Tour de France.
"If there's a good local race, I'm more than happy to jump in," he said.
Even before Monday's announcement, Tour de Georgia officials already had been promoting their race as the potential American farewell for Armstrong.
"While we certainly wish he'd never stop racing, as he has done so much for the sport of cycling and American cycling, we're thrilled and honored that he has chosen the Tour de Georgia as what could be his last professional race here on American soil," said Stan Holm, executive director of the race.
"Lance is a true hero and inspiration to people all over the world. ... We are pleased that the 2005 Dodge Tour de Georgia will be a platform to celebrate Lance's career."
Tour de Georgia officials have received more than 500 media credential applications this year, almost twice last year's total. TV crews with sports, news and entertainment slants attended the press conference.
Armstrong said he went ahead with the retirement announcement Monday, instead of closer to July, so he could "be upfront and honest with the media, the people, the fans, not just here in America and in Europe."
Added Armstrong: "It just seems to me it would be better to announce that and get it out there and let everybody know."
Armstrong said he would remain involved with the Discovery Channel racing team. His new two-year contract to race for the Discovery Channel team requires he race just one more Tour de France. He says he'd like to win a seventh time.
"I was fortunate to win six times. Can I win again this year? I'm not sure, but I'm going to try," he said.
"It's my ambition to win and also a little bit of my job to win," he added.
Armstrong, who overcame testicular cancer, was attracted to the Tour de Georgia's support for the Georgia Cancer Coalition, the official beneficiary of the race.
He has said previously if he retires he would amplify his high-profile role as a cancer survivor.
Armstrong says he is "100 percent committed" to his decision to retire and that he will not be participating in any other races after July 24 — the scheduled end of the Tour de France.
Armstrong's romance with Crow also often places him in gossip magazines and tabloid-style television shows.
"Sheryl, you've been an amazing woman. For someone who is the queen of rock n' roll, you've been a great cycling fan ... a great teammate," he said during the news conference.