The Class D51 (D51形^?) is a type of 2-8-2 steam locomotive built by the Japanese Government Railways, the Japanese National Railways and various manufacturers from 1936 to 1951. The name consists of a "D" for the four sets of driving wheels and the class number 51 for tender locomotives that the numbers 50 through 99 were assigned to under the 1928 locomotive classification rule.

The design of Class D51 was based on the earlier Class D50, which was introduced in 1923. A total of 1,115 D51 locomotives were built, the largest number of locomotives in a single class in Japan. Early D51s built were known as "Namekuji type" (or "slug"). The Class D51 is popularly called "Degoichi" in Japanese.

The specially built D51s that were left on Sakhalin (formerly Karafuto) by the retreating Japanese at the end of World War II were used until 1979 by Soviet Railways. One was left outside Yuzhno-Sakhalinsk railway station, and one is in running condition and is kept at the Yuzhno-Sakhalinsk railway station. Additionally two wrecks were left to the north of the city.