The network announced Tuesday that the actor will star in "Ravenswood," which will premiere in October after the annual "Liars" Halloween episode.
Blackburn's character Caleb Rivers will cross over to the new series.
On his official Twitter account, Blackburn wrote that he's "so stoked" about the role and promises fans "won't be disappointed."
"Ravenswood" will follow five strangers, whose lives become intertwined by a deadly curse that has plagued their town for generations.
"Pretty Little Liars" is about teens whose best friend was murdered in the nearby fictional town of Rosewood, Penn. Since her death they've been continuously harassed via text and pranks by a mysterious figure known as "A." It's based on the best-selling book series by Sara Shepard.
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