O'Brien has starred in "Late Night with Conan O'Brien" in the 12:35 a.m. ET time slot for the past 14 seasons.
Starting next spring, Jimmy Fallon is to step in as the "Late Night" host.
The new late-night lineup was announced Monday by Ben Silverman and Marc Graboff, co-chairmen of NBC Entertainment and Universal Media Studios.
Leno's last "Tonight Show" telecast is to air May 29, 2009.
"'The Tonight Show' has a remarkable history rich in tradition. We're proud of each and every show Jay has shared with America as we look forward to Conan carrying on that outstanding tradition next year," Rick Ludwin, executive vice president of late night and prime time series for NBC Entertainment, said in a statement. "We also can't wait for Jimmy to showcase his winning style following Conan. Jay has left his personal stamp on 'The Tonight Show' for what will be 17 years. He is enormously creative, generous and professional."