The Big Water of Fleet Viaduct is a large, disused single-track viaduct constructed of granite. It was opened by the Portpatrick Railway in 1861. At just over 274 meters (900 feet) long, 21 meters (69 feet) tall, the 20-arch structure was the largest engineering structure on the railway.

The viaduct required much maintenance during its active life, cracks started to appear in the structure, then in 1924-1926 the viaduct piers were encased in red brick which contributes the viaducts current striking appearance.  Old rails were used to strengthen the arches. During World War II, millions of tonnes of ordnance were transported using the viaduct.

The line was closed in 1965 as part of the Beeching Cuts, which were a plan to increase the efficiency of the nationalized rail system. The nearby Little Water of Fleet Viaduct was blown up in 1987 as part of an Army training exercise.

The Big Water of Fleet Viaduct has a cinematic history, appearing in films like The 39 Steps (1935) and Five Red Herrings (1975).