ABC is keeping details of Gish's role top secret for now, but her episode will air in either late April or early May. The casting news was first reported by Zap2it.
The actress, who turns 39 on March 13, joins the ABC drama's diverse mix of upcoming guest stars, a list that includes rapper and music producer Timbaland, Battlestar Galactica's James Callis and a return appearance from Family Guy creator Seth MacFarlane.
Gish most recently appeared on the Showtime series Brotherhood.
FlashForward returns Thursday, March 18 at 8/7c.