The ALCO 251 is a 4-stroke diesel engine that was developed by the American Locomotive Company to replace its 244 and 539 engines. The 251 was developed to be used in diesel locomotives, as a marine power plant in ships, and as a stationary power generator.

The American Locomotive Company was an American manufacturer that operated from 1901 to 1969, initially specializing in the production of locomotives but later diversifying and fabricating at various times diesel generators, automobiles, steel, tanks, munitions, oil-production equipment, as well as heat exchangers for nuclear power plants.