The London Underground D78 Stock, commonly referred to as D Stock, was a type of sub-surface rolling stock which operated on the District line of the London Underground, except on the Wimbledon to Edgware Road service. The first units were withdrawn in January 2015 with the last withdrawn on 21 April 2017.

The D stock was ordered in 1976 to replace the pre-war CO/CP Stock and post-war R Stock on the District line. Seventy-five six-car trains were built by Metro-Cammell, Washwood Heath, the first entering service on 28 January 1980 with last delivered in 1983. The class received the Class 499/0 designation on British Rail's TOPS system to operate on the Richmond and Wimbledon branches.

In 2014 Vivarail purchased 156 driving motor cars and 70 trailer vehicles for conversion to Diesel–electric multiple units. 75 two or three-car units proposed. Under TOPS they are designated as Class 230. Five units have been converted to Class 484 for the Island Line to replace the Class 483 sets.