The actress will play Jane, a 21-year-old widow who serves as a new love interest for Liam (Matt Lanter), according to The Hollywood Reporter.
The pair will first cross paths when Jane moves to Beverly Hills from Alaska, but will find their relationship strained by her guilt over her husband's death.
Liam previously dated Annie (Shenae Grimes) but they parted ways in 90210's third season finale.
Apgar played Lily Smith on The CW's Privileged from 2008 to 2009 and also held a recurring role on Terminator: The Sarah Connor Chronicles.
Her recent television credits include guest roles on recently-axed crime drama Detroit 1-8-7 and CSI Miami.
90210 star Lanter recently revealed that he is looking forward to the show's new college-set storylines.
90210 will return to The CW on September 13 at 8/7c.
> Trevor Donovan to leave 90210
Watch Liam and Annie's break-up on 90210 below: