An SD9 is a 6-axle road switcher diesel locomotive built by General Motors Electro-Motive Division between January 1954 and June 1959. An EMD 567C 16-cylinder engine generated 1,750 horsepower (1.30 MW). Externally similar to its predecessor, the SD7, the SD9 was built with the improved and much more maintainable 567C engine.  Four hundred and seventy-one SD9s were built for American railroads, while a further 44 were produced for export.  Many SD9s both high and short-hood can still be found in service today on shortline railroads and industrial operators. Although most Class 1 roads stopped using these locomotives by the 1970s and 1980s, some remain in rebuilt form on some major Class I railroads, as switcher locomotives.

The SD9 was the second model of EMD's SD (special duty) line of locomotives, following the SD7. Just as the SD7 was a lengthened GP7 with two additional axles, the SD9 was a corresponding modification of the GP9. The additional axles in SD series locomotives provide more tractive effort and more even distribution of locomotive weight compared to the four axle GP series locomotives.  SD9s can be distinguished from the similar looking SD7s by observing the position of the classification lights on the ends of the locomotive, above the number board. The SD9's classification lights are on a small pod, canted outward, while the SD7's classification lights are closer to the centerline and flush with the hood. The last phase of construction had a carbody similar to the SD18 and SD24, and used two 48-inch (1,219 mm) cooling fans instead of four 36-inch (914 mm) cooling fans.  EMD ended SD9 production in June 1959. It was succeeded by the SD18 which began production in 1960.