Elizabeth suggested earlier this month that she was reluctant to return for a new entry without seeing a script, however Deadline reports that the actress has now signed on for the film.
Elizabeth appeared in both American Pie and American Pie 2 as Czech exchange student Nadia. However, she was absent from 2003's American Wedding.
American Reunion sees all of the original cast members return for their high school, and will explore the events of their lives since Jim and Michelle's marriage.
The film's ensemble cast includes Jason Biggs, Seann William Scott, Thomas Ian Nicholas, Alyson Hannigan, Chris Klein, Tara Reid, Jennifer Coolidge, Mena Suvari, Eddie Kaye Thomas, Shannon Elizabeth, and Eugene Levy as Jim's dad.
The film will be both written and directed by Jon Hurwitz and Hayden Schlossberg, best known for their Harold & Kumar film series.
American Reunion will launch in cinemas on April 6, 2012.
Watch Shannon Elizabeth in the trailer for the original American Pie below: