Chris McKenna is also returning as executive producer, the network said.
NBC fired Harmon at the end of the show's third season, in part, because he wasn't getting along with cast member Chevy Chase, who has since left the show.
The network announced Harmon's rehiring Monday, but did not comment further on the development.
No debut date for the new season has been announced yet.
Starring Jim Rash, Joel McHale, Gillian Jacobs, Yvette Nicole Brown, Danny Pudi, Alison Brie, Donald Glover and Ken Jeong, the show is about a group of students and staff at a community college.