Started to appear in late 1942 and was based on the PzKpfw III Ausf J chassis.
Early models had bolted on supplemental armor on the front, with later models having it welded. Later models had the driver's binocular vision device removed. Late models had the ventilator mounted on the rear wall of the superstructure, whereas the early models had it mounted in the roof.
A new commander cupola with 8 periscopes was added. A folding shield for the machine gun was installed in front of the loader's hatch on the roof.
The sides of the superstructure were slanted and slanted plates were installed to protect the panniers. Most had Schürzen installed to protect against hollow charged weapons. Some also had Zimmerit paint to help against magnetic mines.
The 7.5 cm StuK 40 L/48 gun included a muzzle brake.
The coaxial machine gun wasn't added until early 1944. In late spring 1944 a Nahverteidigungswaffe (close-in defense weapon) and a remote controlled machine gun in the roof was added.
Had commander's cupola with 7 episcopes. No box shaped projections on the sides of the superstructure. 165 PzKpfw III Ausf M chassis were used for the StuG Ausf G and 173 PzKpfw III that were returned for overhaul during 1944 were converted as well.