Lanarkshire, also called the County of Lanark (Scottish Gaelic: Siorrachd Lannraig; Scots: Lanrikshire), is a historic county, lieutenancy area and registration county in the Central Lowlands and Southern Uplands of Scotland. The county is no longer used for local government purposes, but gives its name to the two modern council areas of North Lanarkshire and South Lanarkshire.

The county was established as a shire (the area controlled by a sheriff) in the twelfth century, covering most of the basin of the River Clyde. The area was sometimes known as Clydesdale. In the early fifteenth century the western part of the shire was removed to become Renfrewshire. The historic county of Lanarkshire includes Glasgow, but the city had a separate lieutenancy from 1893. A Lanarkshire County Council existed from 1890 until 1975, which was based in Glasgow until 1964 when it moved to Hamilton.

Lanarkshire is generally bounded to the north by Dunbartonshire and Stirlingshire, to the north-east by West Lothian and Midlothian, to the east by Peeblesshire, to the south by Dumfriesshire, and to the west by Ayrshire and Renfrewshire. The county is more rural in the south where it extends into the hills of the Southern Uplands, and more built-up in the north where it includes much of the Greater Glasgow conurbation, Scotland's largest urban area.

It is not known exactly when the shire of Lanark was created; it seems likely that it was created by David I (reigned 1124–1153) who was generally responsible for introducing shires as part of his introduction of Anglo-Norman style administration to Scotland. However, the first documented reference to Lanarkshire dates from the reign of David's successor, Malcolm IV (reigned 1153–1165).

Lanarkshire covered much of the basin of the River Clyde, and took its name from the town of Lanark, which is near the geographic centre of the county and served as the first county town. When first created the shire included on its western side an area south of the Clyde stretching out to the Firth of Clyde. This western area was made the separate county of Renfrewshire in the early fifteenth century. The remainder of Lanarkshire was then divided into two wards, each with their own sheriff-substitute: Over Ward was administered from Lanark, and Nether Ward was administered from Rutherglen and Hamilton at different times. In the mid-eighteenth century the two wards were replaced with three wards: Upper Ward based in Lanark, Middle Ward based in Hamilton, and Lower Ward based in Glasgow.

Elected county councils were created in 1890 under the Local Government (Scotland) Act 1889. Lanarkshire County Council covered the combined area of the three wards, but excluding the three burghs of Airdrie, Glasgow and Hamilton, which were deemed capable of running their own affairs. Glasgow was subsequently made a county of itself in 1893, also removing the city from the rest of Lanarkshire for judicial and lieutenancy purposes. Reforms to county council powers in 1930 saw Airdrie and Hamilton brought within the administrative area of the county council.

Lanarkshire County Council was initially based at County Buildings in Wilson Street in Glasgow; whilst the city was outside the area controlled by the county council, it was found to be the most accessible meeting place for the councillors. From 1930 the county council was based at Lanarkshire House in Ingram Street in Glasgow. Lanark County Council relocated to Lanark County Buildings in Hamilton in 1964.

In 1975, the county council was abolished under the Local Government (Scotland) Act 1973, which established a new system of upper-tier regions and lower-tier districts. Lanarkshire was included within the Strathclyde region, and the county council's former area was divided between seven districts: