The 24-year-old actor will play Eric Kahn, brother of Noel (Brant Daugherty), according to TVLine.
The character will make his debut in the third season's ninth episode, due to air on August 7.
Amell's past credits include guest roles on Alcatraz, CSI: NY and How I Met Your Mother.
He also signed up for a role in Fox comedy pilot Like Father from Scrubs creator Bill Lawrence, but the project was not picked up to series.
Vanessa Ray was previously cast on Pretty Little Liars - the Suits star will play Cece, an ex-girlfriend of Jason (Drew Van Acker) and former mentor of the late Alison (Sasha Pieterse).
Pretty Little Liars airs on Tuesdays at 8/7c on ABC Family. In the UK, the series airs on MTV.
> Pretty Little Liars exec teases 'formidable, familiar villains'
Watch a trailer for season three of Pretty Little Liars below: