The 59-year-old actor will appear in a recurring role as Irving Carver, the "witty, sophisticated attorney" of billionaire Reese Dipple, who is played by Oliver Platt, Deadline reported.
Carver is described as a man who prefers civilised and intelligent debate over mindless bullying, and a man who is willing to use whatever it takes to assure that Lockhart, Agos & Lee is working to further his client's agenda.
Following his role as public defender Sandy Cohen on "The OC", Gallagher has more recently starred in action drama "Covert Affairs" and had a recurring role on HBO's "Togetherness" and NBC's "Law & Order: SVU".
The new characters will fill the places left by popular cast members Matthew Goode and Archie Panjabi, who both left the series at the end of season six.
"The Good Wife" returns on October 4 on CBS in the US.