Zimmer, 43, has recently inked a deal to return as super agent Ari Gold's (Jeremy Piven) sometimes-rival in writer-director Doug Ellin's film, according to The Hollywood Reporter.
Her casting comes after reports emerged that Billy Bob Thornton is in talks to join the television spin-off.
'Entourage's main cast of Jeremy Piven, Adrian Grenier, Kevin Connolly, Jerry Ferrara and Kevin Dillon are all returning for the upcoming movie.
Production on the 'Entourage' film began earlier this month.
The movie is due for release next year.