The County of Nairn, or Nairnshire, (Scottish Gaelic: Siorrachd Inbhir Narann) is a historic county, registration county and lieutenancy area of Scotland. The county was named after Nairn, its only town. The county was used for local government until 1975 when the area was redesignated as the Nairn District, one of the eight districts of the two-tier Highland region. Nairn district was abolished in 1996 when Highland became a single-tier council area.

The county borders Inverness-shire to the west and south, Moray to the east, and has a coastline along the Moray Firth to its north.

The area that would become Nairnshire was anciently part of the Province of Moray, ruled by the Earls or Mormaers of Moray, sometimes as a vassal of the Scottish crown, and at other times as a separate kingdom. The province was finally absorbed into the Kingdom of Scotland during the 12th century. In order to secure the Scottish crown's authority over the area, it was divided into shires, being areas administered by a sheriff. The old province of Moray was broadly split into the three shires of Elginshire, Nairnshire, and the mainland parts of Inverness-shire.

It is not known exactly when the shire of Nairn, administered by the Sheriff of Nairn, was created; there is some evidence that the area was initially included in Inverness-shire, but from the reign of William the Lion (reigned 1165–1214) there was a separate shire of Nairn. There was also a short-lived shire to the east centred on Forres, which in the early 14th century was grouped with Nairn, before later being united instead with Elginshire.

Over time, Scotland's shires became more significant than the old provinces, with more administrative functions being given to the sheriffs. In 1667 Commissioners of Supply were established for each shire, which would serve as the main administrative body for the area until the creation of county councils in 1890. Following the Acts of Union in 1707, the English term 'county' came to be used interchangeably with the older term 'shire'.

Elected county councils were established in 1890 under the Local Government (Scotland) Act 1889, taking most of the functions of the commissioners (which were eventually abolished in 1930). Nairnshire County Council held its first meeting on 22 May 1890 at the Town and County Buildings in Nairn, the county's courthouse (built 1818) which also served as the meeting place for the commissioners and for the town council of the burgh of Nairn.

The 1889 Act also led to a review of boundaries to eliminate exclaves. There were several such changes affecting the boundaries of Nairnshire, which had five detached parts: three small areas surrounded by Elginshire, a larger area of moorland at Dunmaglass surrounded by Inverness-shire, and the small village of Urquhart on the Black Isle, surrounded by Ross and Cromarty. Each of these areas was transferred to the county which surrounded them. Parish and county boundaries were also adjusted such that each parish was in a single county, with the exception of the parish of Croy and Dalcross, which was exceptionally allowed to continue to straddle Inverness-shire and Nairnshire. The boundary changes all took effect in 1891.

In 1927 Nairnshire, along with many of Scotland's other counties, was granted a coat of arms by the Lord Lyon King of Arms, the governor of Scotland's heraldry. The arms was as follows: Or, a chevron gules, between two water-bougets in chief and a stag's head cabossed in base sable, three mullets argent. The motto was "unite and be mindful".