The actress has landed a recurring role in the CW show, TV Line says.
Her character Alana is a member of the sorority that Naomi (AnnaLynne McCord) joins at California University.
Alana is described as intelligent and a natural leader. She is also thought to have a large vocabulary.
Hagan previously played Millie in Freaks and Geeks and starred as Amanda in the final season of Buffy The Vampire Slayer.
She has also had guest roles in shows including Grey's Anatomy, Medium and Private Practice.
Other stars who will appear in the fourth season of 90210, which will premiere on September 13, include Matt Cohen, Justin Deeley, Megalyn Echikunwoke and Kristina Apgar.