"Congratulations, Jimmy," said Leno, who has been "Tonight" emcee for 22 years in Burbank, Calif. "I hope you're as lucky as me and hold on to the job until you're the old guy. If you need me, I'll be at the garage."
"Jay Leno is an entertainment icon, making millions of people laugh every weeknight for more than 20 years," NBCUniversal Chief Executive Officer Steve Burke said in a statement. "His long reign as the highest-rated late-night host is a testament to his work ethic and dedication to his viewers and to NBC. We are purposefully making this change when Jay is No. 1, just as Jay replaced Johnny Carson when he was No. 1. Jimmy Fallon is a unique talent and this is his time. I'm thrilled he will become the sixth host of 'The Tonight Show' at exactly the right moment, in conjunction with our coverage of next year's Winter Olympic Games from Sochi, Russia."
"I'm really excited to host a show that starts today instead of tomorrow," Fallon, 38, said.
The announcement ended weeks of speculation regarding who will replace Leno, 62, and when -- as well as whether Leno's departure was his idea or NBC's.
The network previously booted Leno as host of "Tonight" at the end of the 2008-09 season, despite high ratings, and gave the position to "Late Night" host Conan O'Brien to keep O'Brien from moving to another network, and to court younger viewers in the same time slot.
NBC then offered Leno a nightly, hourlong, 10 p.m. series, which kicked off in September 2009.
But when the prime time "Jay Leno Show" failed to be a strong lead-in for local news broadcasts, NBC announced it would push Leno's show back to 11:35 p.m., shorten it to 30 minutes and air "Tonight" at 12:05 a.m. However, O'Brien said he would not remain on "Tonight" if it was bumped to 12:05 a.m. and left the network, paving the way for Leno's return as "Tonight" host in the traditional 11:35 p.m. time slot in March 2010.
O'Brien now heads up his own chat show "Conan" on TBS, which announced Monday it has renewed the show through November 2015.
Fallon's former boss at "Saturday Night Live" is to serve as executive producer of "The Tonight Show" when it moved back to its original home of 30 Rockefeller Center.
NBC said programming plans for the 12:35 a.m. time period are in development and will be announced soon. Fallon's "Late Show" now occupies that slot.