Sanquhar is a town on the River Nith in Dumfries and Galloway, Scotland, north of Thornhill and west of Moffat. It is a former Royal Burgh. It is notable for its tiny post office, established in 1712 and considered the oldest working post office in the world. It was also where the Covenanters, who opposed episcopalisation of the church, signed the Sanquhar Declaration renouncing their allegiance to the King, an event commemorated by a monument in the main street.