Grey's Anatomy Welcomes New Cast Member
07/11/2007 AT 01:00 PM EDT
Leigh, 25, will be a series regular, playing Meredith's half sister, Lexie Grey – a new intern at Seattle Grace, a rep for ABC Television Studios confirms to PEOPLE.
Leigh first appeared in the last two episodes of the previous season, where her character's chemistry with Derek Shepard (Patrick Dempsey) caused tension between him and Meredith (Ellen Pompeo).
She previously starred in the short-lived series Reunion and That 80s Show and has appeared on The Practice and 7th Heaven. She also starred in 2001's Not Another Teen Movie.
Leigh joins the cast on the heels of Isaiah Washington's dismissal from the show on June 9, when he was informed that ABC was not picking up his option to return to the hit series.
The Grey's Anatomy cast began filming the show's fourth season on Wednesday after a two-month hiatus.
News of Leigh's possible hiring was first reported by USA Today.