Derby Line is an incorporated village in the town of Derby in Orleans County, Vermont, United States, slightly north of the 45th parallel, the nominal U.S.-Canada boundary. The population was 673 at the 2010 census.
The village is located on the Canada–United States border and is contiguous with the district of Rock Island in the town of Stanstead, Quebec.
Notable buildings include the Haskell Free Library and Opera House.
The village was incorporated in 1791. It lies on an elevation at the far north of Derby, which was chartered on October 29, 1779 and first settled in 1795. By 1859, the area was noted for the beauty of its farmhouses and luxuriant farmland, set before the distant vista of Lake Memphremagog and the Green Mountains.
Because of an erratic survey, the border separating Canada from the United States was drawn incorrectly by the surveyors in the 18th century, above the 45th parallel which was the agreed boundary. Derby Line was founded based on that line, above the 45th parallel. The boundary was confirmed by the Webster–Ashburton Treaty in 1842.
Derby Line is known for the Haskell Free Library and Opera House, a line house deliberately constructed on the international border and opened in 1904. The donors were a binational couple: Carlos F. Haskell was a local American businessman who owned a number of sawmills, while Martha Stewart Haskell was Canadian.