The Great Western Railway (GWR) 3100 Class was a class of 2-6-2T side tank steam locomotive.

This class of large prairie was created in 1938 when Collett rebuilt some of Churchward's 3150 Class engines for use as bankers, particularly from Severn Tunnel Junction shed. These engines used the same standard class 4 boiler, but with pressure increased to 225 psi (1,550 kPa) and smaller 5 feet 3 inches (1.600 m) driving wheels and a one-half inch (13 mm) increase in cylinder diameter. This raised their tractive effort to 31,170 pounds (14,140 kg). Almost unnoticeable was a 2 in (50.8 mm) reduction in pony truck wheel diameter to 3 ft 0 in (914 mm). Only five engines were ever modified, Nos. 3173, 3156, 3181, 3155 and 3179 which were rebuilt as 3100 to 3104 respectively. All members of the class were scrapped.

• GWR 3100/5100 Class (1906)
• GWR 3150 Class
• GWR 5101 Class
• GWR 6100 Class
• GWR 8100 Class
• List of GWR standard classes with two outside cylinders

• Whitehurst, Brian (1973). Great Western Engines, Names, Numbers, Types and Classes (1940 to Preservation). Oxford, UK: Oxford Publishing Company. pp. 30, 102, 135. ISBN 978-0-9028-8821-0. OCLC 815661.

• Class 3100 Details at Rail UK