The kit is available at authorized Ford dealers and is priced at $999.
It turns the track-oriented Mustang into a four-seater.
"We always listen to customer feedback," said Dave Pericak, Ford Performance global director, in a statement. "So when many requested a backseat for their GT350R, we acted quickly to make it available."
The kit features Alcantara material and red accent stitching.
The Shelby GT350R competes against the Chevrolet Camaro Z/28.
Edmunds says: Shelby GT350R buyers with a practical bent should be eager to order this new backseat kit.