Gallagher, 59, will play Irving Carver, a witty and sophisticated attorney representing conservative billionaire Reese Dipple (Oliver Platt), said The Hollywood Reporter.
Irving favors a civilized and intelligent debate over bullying but is not afraid to do whatever it takes to make sure the firm is pushing his client's agenda forward.
Platt recurred last season on the series, sharing many scenes with Diane (Christine Baranski).
Gallagher joins Margo Martindale and Jeffrey Dean Morgan as new additions for the upcoming season.
The Good Wife's new season premieres at Sunday, October 4 on CBS.