She will play the recurring role of Zoey, a criminal defense attorney who will have a romantic arc on the show, the network said.
"We're thrilled that Gabrielle is joining our cast," executive producer David S. Goyer said in a statement. "When we met with her, we immediately knew she was our Zoey. She's witty, soulful and beautiful. I've been wanting to work with her for a long time."
"FlashForward" is about what happens when the world's population is given a glimpse of their future.
Union's film credits include "Cadillac Records," "The Perfect Holiday," "Daddy's Little Girls," "Running with Scissors," "Bad Boys II," "Breakin' All the Rules" and "Bring it On." On television, she was recently seen on "Life" and "Ugly Betty."