The chat series will air weeknights from 11:35 p.m. to12:35 a.m.
"Late Night with Jimmy Fallon" will continue to be broadcast from 12:35 to 1:35 a.m.
The announcements were made by Jeff Gaspin, chairman of NBC Universal Television Entertainment.
"We're pleased that Jay is returning to host the franchise that he helmed brilliantly and successfully for many years," Gaspin said in a statement. "He is an enormous talent, a consummate professional and one of the hardest-working performers on television."
Leno previously served as host of "The Tonight Show with Jay Leno" from May 1992 through May 2009.
NBC ousted him as host at the end of last season and gave the position to Conan O'Brien to keep O'Brien from moving to another network. NBC then gave Leno his own nightly, hourlong, 10 p.m. chat series, "The Jay Leno Show." When the prime time series failed to be a strong lead-in for local news broadcasts, however, 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 last week he will not remain on "Tonight" if it is moved to 12:05 a.m. He reportedly had been locked in intense negotiations with the network since.
The Wall Street Journal said O'Brien signed a $44 million deal early Thursday to leave NBC.
O'Brien is believed to be getting $32 million, while about $12 million will go toward severance packages for his staff, the Journal said.
NBC confirmed it reached an agreement with O'Brien, but did not offer any specifics about the financial arrangements.
"Under terms of an agreement that was signed earlier today, NBC and O'Brien will settle their contractual obligations and the network will release O'Brien from his contract, freeing him to pursue other opportunities after Sept. 1, 2010," the network and O'Brien said in a joint statement.
O'Brien's final appearance as host of "The Tonight Show" is set for Friday, the network said.