Series creator Shonda Rhimes recently confirmed the casting on her Twitter feed.
She wrote: "We have Nicholas Brendon (yes, that is Xander from Buffy) guesting on PP today! Super nice guy and very fun to work with!"
Brendon played Xander Harris for seven seasons between 1997 and 2003. More recently he starred in an episode of Without a Trace and played a recurring role on Criminal Minds.
The fourth season of Private Practice begins on September 23 on ABC.