The How I Met Your Mother star played band geek Michelle Flaherty in the original three films. Deadline reports that she has joined the rest of the cast for American Reunion.
Universal previously revealed details of the fourth true instalment of the comedy series, including the names of returning cast members. The likes of Jason Biggs, Seann William Scott, Tara Reid and Chris Klein were on the list, while Hannigan was not originally.
The new film will reportedly revolve around the original group arranging a ten-year reunion via Facebook. It will show how their lives have changed since 2003's American Wedding.
Producer Craig Perry said last year that he hoped for a full cast to return to the series.
American Reunion will arrive in cinemas in summer 2012. The series has spawned many direct-to-DVD sequels.
Watch the trailer to the original American Pie below: