The 36-year-old Armstrong told Vanity Fair in an exclusive interview posted on its Web site Tuesday that he was inspired to return after finishing second last month in the Leadville 100, a lung-searing 100-mile mountain bike race through the Colorado Rockies.
"This kind of obscure bike race, totally kick-started my engine," he told the magazine. "I'm going to try and win an eighth Tour de France."
The sport and particularly the Tour have missed his star power, even though skeptics refused to believe he could win 7 Tours without the help of illegal performance-enhancing drugs.
The 2009 Tour "is the intention," Armstrong's spokesman Mark Higgins told The Associated Press, "but we've got some homework to do over there."
Tour director Christian Prudhomme did not return messages seeking comment on Armstrong's decision. His staff said he would not comment before Wednesday morning, if at all.
Armstrong's close friend and longtime team director, Johan Bruyneel, now with team Astana, sent a text message to an AP reporter in Paris saying he did not want to comment now.
In a video statement on his foundation's Web site, Armstrong said details - such as a team and schedule - will be announced Sept. 24 at the Clinton Global Initiative in New York City.
"I am happy to announce that after talking with my children, my family and my closest friends, I have decided to return to professional cycling in order to raise awareness of the global cancer burden," Armstrong said in a statement released to The Associated Press. "This year alone, nearly eight million people will die of cancer worldwide. ... It's now time to address cancer on a global level."
In the Vanity Fair interview, Armstrong told the magazine he's 100 percent sure he's going to compete in the Tour next summer.
"We're not going to try to win second place," Bill Stapleton, Armstrong's lawyer and longtime confidant, told the AP.
"I think it's great," said longtime teammate George Hincapie, who added he spoke to Armstrong on Tuesday morning. "Like I said earlier today without Lance half the teams in this race probably wouldn't be around. He's done more than anyone for the sport especially in America and around the world."
"On a personal note, I like that he's going to be back in the peloton. He's a great friend of mine, and I also think for the sport it's good, too."
Armstrong noted in the magazine interview that other athletes in his age range are competing at a high level, specifically 41-year-old Olympic medalist swimmer Dara Torres and 38-year-old Olympic women's marathon champion Constantina Tomescu-Dita, of Romania.
"Older athletes are performing well," he said. "Ask serious sports physiologists and they'll tell you age is a wives' tale."
Age will be an issue for Armstrong in the Tour de France. He'll be 37 next week, ancient for such a grueling competition. Only one rider older than 34 has ever won the Tour - 36-year-old Firmin Lambot in 1922.
On Monday, the cycling journal VeloNews reported on its Web site that Armstrong would compete with the Astana team, led by Bruyneel, in the Tour and four other road races - the Amgen Tour of California, Paris-Nice, the Tour de Georgia and the Dauphine-Libere.
But there are no guarantees Astana would be allowed to race in the 2009 Tour. Race officials kept the team out of the 2008 Tour because of previous doping violations.
If Armstrong and his team aren't invited in 2009, he plans to appeal directly to French President Nicolas Sarkozy.
"I've already put a call in to him," he told Vanity Fair.
Armstrong's return to competition raises the question of whether he risks damaging his athletic legacy. And his own words likely will cause some to wonder if he'll approach his return with the same steely-eyed determination and passion.
In an interview published in the October issue of Men's Journal, Armstrong said, "I'm glad I'm not cycling anymore ... It was fun while it lasted, and I liked it, but I'm so focused on other things now that I never think about it."
He's certainly thinking about it now.
With his riveting victories over cancer and opponents on the bike, to his work for cancer awareness and gossip-page romances, Armstrong has become a modern-day American icon.
He was an established sprint champion when he was diagnosed in 1996 with testicular cancer that had spread to his lungs and brain.
Doctors gave him less than a 50 percent chance of survival.
Surgery - he has a half-moon scar on his head from the brain operation - and brutal cycles of chemotherapy saved his life. From there, it was determination and powerful self-discipline that led him back to the bike.
His stunning win at the 1999 Tour de France was just the start.
Under the guidance of close friend and U.S. Postal Service team director Bruyneel, Armstrong morphed from a sprinter into a technical expert who could climb mountains at speeds that punished other riders.
Armstrong's goal every year was to win the Tour de France, the sport's biggest race, and he dominated the Pyrenees and Alps like no other rider ever had.
The victories also forced him to defend himself against skeptics who questioned whether he was cheating by using performance-enhancing drugs. He got in several public spats with officials at the World Anti-Doping Agency.
"There's this perception in cycling that this generation is now the cleanest generation we've had in decades, if not forever," Armstrong told Vanity Fair. "And the generation that I raced with was the dirty generation."
Although many riders were caught doping, Armstrong never tested positive and has always maintained he was a clean rider, using hundreds of passed drug tests during his career as proof.
His Lance Armstrong Foundation has raised hundreds of millions of dollars for cancer awareness and survivorship. The foundation's yellow "Livestrong" wristbands that started selling in 2004 are still seen everywhere - with many copycats.
He retired after his 2005 Tour de France victory, diving head first into making cancer a political issue and causing some to ask if he may someday run for office himself.
"This is a damn war for me. It's nothing other than that," Armstrong told The Associated Press in 2007. "I had the disease and I hate it and I hate that we haven't made enough progress against it."
Armstrong has lobbied for cancer treatment funding in Washington, D.C., co-hosted televised cancer forums with presidential candidates and was instrumental in 2007 persuading the Texas Legislature to approval a $3 billion fund for cancer research. He can rally millions of his "Livestrong Army" through his Web site to support cancer causes.
His social life has done just as much to keep him in the spotlight.
After his divorce from wife Kristin, the mother of his three children, Armstrong has had high-profile relationships with rocker Sheryl Crow, fashion designer Tory Burch and most recently, actress Kate Hudson.
