Ayrshire, historic county, southwestern Scotland. The county is named for Ayr, its historic county town (seat). Apart from a small section in the south that is part of the council area of Dumfries and Galloway, Ayrshire is presently divided into the council areas of South Ayrshire, East Ayrshire, and North Ayrshire.

The county has a concave western coastline along the Firth of Clyde and rises to an elevation of about 2,000 feet (600 metres) at its eastern boundary in the Southern Uplands. Rivers divide it into three ancient regions: Cunninghame, north of the River Irvine; Kyle, between the Irvine and Doon; and Carrick, south of the Doon.

Archaeological finds show that Ayrshire has been occupied for thousands of years. A Roman fort (79–84 ce) survives at Loudoun Hill. During the early Middle Ages, Ayrshire was part of the rudimentary Celtic British kingdom of Strathclyde. In the 11th century its king Duncan became the first ruler of all Scotland. Invading Norwegians were defeated at the Battle of Largs in 1263, and Sir William Wallace began the struggle to regain Scotland’s independence in 1297 at Ayr. From Turnberry Castle (1307) Robert the Bruce began his fight for the Scottish throne, and in 1315 he held Parliament in Ayr; there Oliver Cromwell later built a citadel (1654) to control southwestern Scotland.

Scotland’s first deep coal mines were sunk in Ayr in 1780, and ironworks and textile industries followed rapidly. Simultaneously the agricultural revolution brought enclosures, draining, and crop rotation, and Ayr became an important and highly developed agricultural area. Its lush lowland pastures have the heaviest stock-carrying capacity in Scotland. The main industrial areas were Irvine and Kilmarnock, in which textiles of all kinds were produced, but during the late 20th century the textile industry and metalworking declined. Engineering and electronics sectors have developed in the county.