Thornhill is a village in the Mid Nithsdale area of Dumfries and Galloway, Scotland, south of Sanquhar and north of Dumfries on the main A76 road. Thornhill sits in the Nithsdale valley with the Carsphairn and Scaur range to the west and the Lowther hills to the east. It was initially a small village, planned and built in 1717 on the Queensberry Estate on the road linking Dumfries to Glasgow. The Earl of Queensberry initially named the village 'New Dalgarnock' however the name did not achieve popular approval.

Dumfries and Galloway is one of the 32 unitary council areas of Scotland, located in the western part of the Southern Uplands. It is bordered by East Ayrshire, South Ayrshire, and South Lanarkshire to the north; Scottish Borders to the north-east; the English ceremonial county of Cumbria, the Solway Firth, and the Irish Sea to the south, and the North Channel to the west. The administrative centre and largest settlement is the town of Dumfries. The second largest town is Stranraer, located 76 miles (122 km) to the west of Dumfries on the North Channel coast.