The show's network, ABC Family, revealed that future episodes will see the introduction of Caleb and Simone.
Caleb, who is described as a "bad boy", will be played by former Days of our Lives actor Tyler Blackburn.
Meanwhile, Supernatural and Veronica Mars star Alona Tal has signed up to play Simone. She is expected to cause "a lot of drama" for one of the existing characters.
Neither Caleb nor Simone appeared in the Pretty Little Liars novel series.
Pretty Little Liars is expected to return for new episodes in January on ABC Family.