Armstrong is the only man to win six Tours de France
It was rumoured the 34-year-old would skip the event but part of the deal with new backers Discovery Channel was he would race at least one more Tour.
Discovery took over sponsorship of Armstrong's team from US Postal, who funded his first six Tour victories.
The Texan said in a statement on Wednesday that he was "looking forward to achieving a seventh Tour (win)".
Armstrong's decision was welcomed by rival rider Jan Ullrich.
Germany's 1997 Tour champion and five-time runner-up said: "It's good that he is there. The best should be at the Tour."
I am excited to get back on the bike and start racing although my condition is far from perfect
Lance Armstrong
Ullrich's T-Mobile team-mate Andreas Kloden, second to Armstrong in 2004, added: "I always said he would ride. I am glad he's there."
Armstrong is set to open his 2005 campaign in the Paris-Nice race from 6-13 March.
His Tour build-up also includes the prestigious spring classic the Tour of Flanders on 3 April.
Armstrong will then return to the US and defend his title in the Tour de Georgia from 19-25 April and other dates to his schedule may be added.
"I am excited to get back on the bike and start racing although my condition is far from perfect," Armstrong said.
"Johan (Bruyneel, the team's sports manager) and I will evaluate my fitness later this spring and possibly add some races to the calendar."
If Armstrong takes part in such an event it is with just one goal in mind: to win it
Johan Bruyneel
Bruyneel is fully behind Armstrong's bid for a seventh successive title.
"There were two possibilities which had been agreed in the contract with the sponsor," said Bruyneel.
"Either to ride in the Tour this year or the next one. We opted for the most realistic solution.
"It is easier to prepare for a similar goal six months ahead of it than 18 months.
"Lance feels sufficiently strong mentally to race again in the Tour de France, which will be his main target this season.
"And you know, if Armstrong takes part in such an event it is with just one goal in mind: to win it."
Lance Armstrong has announced that he will attempt to win a seventh Tour de France crown in 2005.

The 34-year-old Texan announced his intentions in a statement on Wednesday, ending months of speculation.

Armstrong, who suffered from testicular cancer from 1996 to 1998, has won six straight Tour de France titles; a feat no-one else has achieved. 

This season, Armstrong will be riding with the Discovery Channel Pro Cycling Team, after US Postal Service ended their sponsorship of his team last year.

Five times Tour runner-up and 1997 winner Jan Ulrich said in response to the news, "It's good that he is there. The best should be at the Tour."

Armstrong will also be racing in two other road cycling stage races in France and the U.S. this spring, and will be in a one-day race in Belgium in April. There is the possibility of Armstrong competing in more races this season if he and his team manager believe he is fit enough.