The 27-year-old actress, and daughter of Meryl Streep, will return in the fourth episode of the season.
After working on opposite sides of a murder case last season, Nancy and Alicia (Julianna Margulies) will team up as co-counsels representing a DJ and security team who are being sued for a fatal stampede at a rave concert.
Gummer will next star in Shonda Rhimes' new series Off the Map, about a medical clinic in South America. The series is scheduled to debut midseason on ABC.
The Good Wife's sophomore season premieres on CBS, Tuesday, Sept. 28 at 10/9c.