"Why Have You Left the One You Left Me For" is a song written by Mark True, and recorded by American country music artist Crystal Gayle. It was released in October 1978 as the second single from the album When I Dream. The song was Gayle's sixth number one on the country chart. The single stayed at number one for two weeks (Jan. 26 & Feb. 3, 1979) and spent a total of ten weeks on the country chart.