The Firefly was a class of broad gauge 2-2-2 steam locomotives used for passenger services on the Great Western Railway.

Following the success of the Star class locomotives introduced to the Great Western Railway by Daniel Gooch, Gooch set to work to develop a new class based on North Star, but with larger boilers.

The result was the Fire Fly, later followed by 61 similar locomotives designated the same class.

The original Fire Fly is said to have covered the 30.75 miles (49.49 km) from Twyford to London Paddington in 37 minutes, an average speed of 50 miles per hour (80 km/h), which was unprecedented in 1840.

It is a working replica of the original Fire Fly and is based at Didcot Railway Centre.