The ABC drama stars Jennifer Morrison as Emma, a young woman with a troubled past who moves to a small town where the magic of fairytales appears to be real.
Series writer Jane Espenson has now confirmed via Twitter that Buffy actor Groener will appear on the show.
She wrote: "Guess who just got cast in my first ep of Once Upon A Time?! Buffy connection!"
Espenson later added: "That's right! It's Harry Groener! Can't wait to see him [on the show]!"
Groener played the sinister Mayor Richard Wilkins on Buffy and has since appeared in episodes of Bones, Breaking Bad and How I Met Your Mother.
Anastasia Griffith, Paula Marshall and Jessy Schram are among the other stars who have signed up for guest roles in Once Upon A Time.
Once Upon A Time premieres on Friday, October 23 on ABC.
Watch Digital Spy's interview with Once Upon A Time star Jennifer Morrison below: