The actress will play stylist Cece on the ABC Family drama, reports TVLine.
The character will be revealed as an ex-girlfriend of Jason (Drew Van Acker) and former mentor of the late Alison (Sasha Pieterse).
Ray - who has previously appeared in episodes of Damages and White Collar - will make her Pretty Little Liars debut in the seventh episode of season three.
Vampire Diaries actor Sterling Sulieman will also appear in the new season, it was announced last month.
The 28-year-old actor will play Nathan St. Germain, the cousin of Maya (Bianca Lawson).
Pretty Little Liars returns to ABC Family on June 5 at 8/7c. In the UK, the series airs on MTV.
> Pretty Little Liars wins third season
Watch a trailer for the new season of Pretty Little Liars below: