Bell's representative, Marcel Pariseau, says the couple are expecting their first child in late spring.
Shepard, 37, appears on the NBC drama "Parenthood." Bell, 32, has a role on Showtime's "House of Lies," also starring Don Cheadle. She also narrates "Gossip Girl" as the show's title character.
Shepard and Bell co-starred in the film "Hit & Run" this summer which Shepard also wrote and directed.
Earlier this year, Bell joked that she and Shepard had no problem working together because they are "severely co-dependent."
The couple, both originally from Michigan, started dating in 2007. They have been engaged for about three years.
(Copyright 2012 by The Associated Press. All Rights Reserved.)