According to Associated Content, negotiations have been ongoing for two weeks and with only two days left in O'Brien's tenure as presenter, the network has finally made the terms official.
The first episode of Leno's Tonight Show will air on Monday, March 1, back in his original timeslot of 11.35pm. Jimmy Fallon is to follow at 12.35am.
O'Brien's last episode, scheduled to air tomorrow night, is expected to feature the likes of Tom Hanks and Will Ferrell.
However, O'Brien will not be able to return to television until September 1, 2010.