The New York Post reports that Gish will appear in the show's second season.
She is expected to play Anne Sullivan, a mysterious therapist who tries to find out the characters' secrets.
The show's executive producer Marlene King explained: "The police wind up thinking the girls are lying about Ian. [So] their parents get together and insist they see someone to deal with these seemingly obsessive thoughts about pinning Allison's murder on Ian.
"The therapist is going to play a very important part of season two and in the girls' hunt for A."
Gish has previously starred on shows including The X Files, The West Wing and Brotherhood. She also appeared in several episodes of FlashForward.