A diesel locomotive is a type of railway locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel. Several types of diesel locomotive have been developed, the principal distinction being in the means by which the prime mover's mechanical power is conveyed to the driving wheels (drivers).

Early internal combustion engine-powered locomotives used gasoline as their fuel. Soon after Dr. Rudolf Diesel patented his first compression ignition engine in 1892, its application for railway propulsion was considered. Progress was slow, however, because of the poor power-to-weight ratio of the early engines, as well as the difficulty inherent in mechanically applying power to multiple driving wheels on swiveling trucks (bogies).

Steady improvements in the diesel engine's design (many developed by Sulzer Ltd. of Switzerland, with whom Dr. Diesel was associated for a time) gradually reduced its physical size and improved its power-to-weight ratio to a point where one could be mounted in a locomotive. Once the concept of Diesel-electric drive was accepted the pace of development quickened, and by 1925 a small number of diesel locomotives of 600 horsepower were in service in the United States. In 1930, Armstrong Whitworth of the United Kingdom delivered two 1,200 hp locomotives, using engines of Sulzer design, to Buenos Aires Great Southern Railway of Argentina.