The U.S. Open champion is the 22nd player to top the rankings since they were introduced in 1973 and the sixth American-born player after Andre Agassi, Pete Sampras, Jim Courier, John McEnroe and Jimmy Connors.
Roddick beat Spain's Tommy Robredo 6-3, 6-4 in the last 16 to topple Juan Carlos Ferrero from the top spot but he was not aware the number one ranking was in play until after the match.
"Pretty cool," he said when asked for his reaction. "I've got there and no one can take that away from me -- for ever.
"It never crossed my mind when I was a kid that I might be number one. It's definitely something I never thought would happen."
Roddick's elevation to the summit followed Ferrero's shock 7-5, 7-5 defeat by Czech Jiri Novak earlier on Thursday.
The American -- the fourth youngest number one -- now leads the Champions Race and will be confirmed as number one in the 12-month Entry System rankings next Monday.
BULLET DELIVERIES
Roddick's serve was too much for Robredo, seeded 16th, to handle as the 21-year-old's bullet deliveries repeatedly thwacked into the backboards on the centre court in Paris.
The second seed kept the pressure on by hurrying through his serves and broke Robredo's delivery in the fourth game of the first set and the third of the second.
"I definitely cleaned up some things today. I was getting good heat on my serve and I played the big points pretty decently," he said.
After this tournament, Roddick and Ferrero travel to Houston for the season-ending Masters Cup, starting on November 8, where the year-end number one will be crowned.
Also still in with a chance is Wimbledon champion Roger Federer. The Swiss third seed saved four match points before overcoming Dutchman Martin Verkerk 6-7, 7-6, 7-6.
Federer will take on Britain's Tim Henman, who trounced Gustavo Kuerten 6-4, 6-2, for a place in the last four.
If the seedings work out, Federer and Roddick should meet in the semi-final.
Despite his achievement, Roddick said reaching number one was not as important to him as winning in Paris and Houston.
"Every match win or lose could be the difference," said Roddick who will next play Sweden's Jonas Bjorkman, a 6-3, 7-6 winner over French qualifier Gregory Carraz.
"What's done is done but I'd like to be greedy and go for more."
FERRERO OUTPLAYED
Ferrero, who became number one in September, was outplayed from the back of the court by 14th seed Novak.
His forehand, the weapon that won him the French Open this year, misfired throughout.
"I'm surprised but I was playing great tennis today," said Novak. "I had nothing to lose -- this is one of the best results of my life."
Before this tournament Ferrero led the year-long ATP Champions Race by four points from Roddick and he still hopes to end 2003 as top dog. The rankings and the Champions Race converge as the year nears its end.
"He (Roddick) has to play very well here and in Houston to take number one. I will fight with all my powers to get it," said Ferrero.
David Nalbandian of Argentina became the eighth and last player to qualify for the Masters Cup after Thailand's Paradorn Srichaphan was beaten 7-6, 6-3 by qualifier Hicham Arazi of Morocco.
Arazi now plays Novak.
Fifth seed Rainer Schuettler beat Chile's Nicolas Massu 6-4, 7-5.
The Australian Open runner-up next plays Romanian Andrei Pavel whose opponent, fourth seed Guillermo Coria of Argentina, withdrew with a fever.
