The 30-year-old actor — whose credits include playing the villain on Alias, the villain's son on 24 and the heroine's father, also a villain, on The Vampire Diaries — will play a doctor who goes on a date with Mary Margaret (Ginnifer Goodwin), aka Snow White.
Anders first tweeted the news, joking about the secrecy: "Can't tell u anymore as it's a secret so SHHH &/or tell the world." He will first appear in Episode 3.
The series, executive-produced by Lost's Edward Kitsis and Adam Horowitz, stars Jennifer Morrison as Emma, Snow White's long-lost daughter. Emma returns to the small town of Storybrooke, where her son, whom she gave up for adoption, is being raised by the Evil Queen (Lana Parrilla), and every fairy-tale character has been frozen in time.
Once Upon a Time will premiere Sunday, Oct. 23 at 8/7c on ABC.