Nithsdale (Srath Nid in Scottish Gaelic), also known by Scoticised names Strathnith, Stranith or Stranit, is the strath or dale of the River Nith in southern Scotland, and the name of the local area. The name Strath Nid may represent the Cumbric Ystrad Nidd; Cumbric (a variety of Common Brittonic) was the dominant language in this area from before Roman times until the 11th or 12th centuries, whereas Gaelic influence here was late and transient. The River Nith flows north to south through the Southern Uplands in south-west Scotland, separating the Lowther Hills from the Scaur Hills. Nithsdale has historically been a strategic area as it forms an invasion route from England into central Scotland.
Nithsdale was also a historic district of Scotland, bordering Annandale to the east, Clydesdale to the north, Kyle to the north-west and Galloway to the west. The district was in the Sheriffdom of Dumfries and later became part of Dumfriesshire, one of the counties of Scotland. The Local Government (Scotland) Act 1889 established a uniform system of county councils in Scotland, and realigned the boundaries of many of Scotland's counties: Nithsdale became one of three sub-divisions of Dumfriesshire, along with Eskdale (previously part of Liddesdale) and Annandale.
- The burghs of Dumfries and Sanquhar
- The Districts of Thornhill and Upper Nithsdale
- Most of the District of Dumfries
- The Eastern District of the County of Kirkcudbright