Steve Burke, chief executive officer of NBC Universal, said the network was making the move when Leno was still atop the ratings, just as Johnny Carson was when Leno replaced him in 1992. "Jimmy Fallon is a unique talent and this is his time," Burke said.
Leno, in a statement, offered his congratulations to Fallon: "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."
Fallon said: "I'm really excited to host a show that starts today instead of tomorrow."
NBC has been quietly building a new studio for Fallon at its Rockefeller Center headquarters. Tonight began in New York in the 1950s, but Carson moved it to California in 1972.
A storied part of television tradition, the network late-night shows find themselves with much more competition now with cable lineups like Adult Swim, smaller talk shows hosted by the Comedy Central duo of Stewart and Colbert, and a device - a large number of people spend that time of night watching programs they recorded on their DVRs.
NBC worries that ABC's Jimmy Kimmel will establish himself as a go-to late-night performer for a younger generation if it doesn't move swiftly to install Fallon at Tonight. ABC moved Kimmel's time slot to directly compete with Leno earlier this year.
But the move also has the potential to backfire with Leno's fans, who did not embrace Conan O'Brien when Leno was temporarily moved to prime time a few years ago.
NBC has long prided itself on smooth transitions, but that reputation took a hit with the short-lived and ill-fated move of O'Brien to Tonight.
The Leno-Fallon changeover didn't begin smoothly. Leno had been cracking jokes about NBC's prime-time futility, angering NBC entertainment chief Robert Greenblatt, who sent a note to Leno telling him to cool it. That only made Leno go after NBC management harder.
The first public effort toward making the transition smooth was Monday night, when Leno and Fallon appeared in a comic video making fun of the late-night rumors. It aired in between each man's show.
John Dawson, general manager for five NBC affiliates that have extensive reach throughout Kansas, said it would be difficult to give up a program that wins its time period by 33 percent.
But he said he believed in Fallon and in NBC's corporate owner, Comcast, the nation's largest cable company.
"Comcast certainly knows how to launch entertainment programming," Dawson said.