Day-Vandevander Mill, also called the Old Mill at Harman, is a historic grist mill near Harman in West Virginia. Built about 1877, it survives as a mill museum and was listed on the National Register of Historic Places in 1987. Its original hand-hewn post-and-beam building uses mortise-and-tenon joinery and reflects 19th-century mill construction, while additions from the 1920s show later changes through studded wall framing. The site also preserves its mill pond and tail race, important features of the water-powered milling landscape and evidence of how the property developed over time.