Lifetime spokeswoman Kannie Yu LaPack says Alba will serve as a guest judge on Thursday's episode of the popular clothing-design competition show.
The 28-year-old actress will help judge a challenge in which the remaining six contestants must create an outfit for an "opinionated celebrity," who they will later discover is show host Heidi Klum.
Other guest judges this season have included Nicole Richie, Molly Sims and Lauren Hutton.
