Almarian William Decker (born 1852, Ohio; d. Aug. 1893, Sierra Madre, California; interred Sierra Madre Pioneer Cemetery) was an American pioneer of electrical engineering involved in the early development of three-phase electrical power. In 1892 he was hired by H. H. Sinclair and Henry Fisher of the Redlands Electric Light and Power Company, a Californian generating company, to design a new three-phase generator for the Mill Creek No. 1 hydroelectric plant. The plant opened in 1893 and is still in operation today (2026). This was the first commercial application of three-phase electrical power in the United States and probably the world. Its success led to the widespread adoption of three-phase power, in preference to single-phase and direct current.
Decker was also hired by Prof. Thaddeus Lowe of the Mount Lowe Railway which opened in Altadena, California in 1893.