Caithness (/keɪθˈnɛs/; Scottish Gaelic: Gallaibh [ˈkal̪ˠɪv]; Old Norse: Katanes) is a historic county, registration county and lieutenancy area of Scotland.

There are two towns, being Wick, which was the county town, and Thurso. The county includes the northernmost point of mainland Britain at Dunnet Head, and also the most north-easterly point at Duncansby Head near John o' Groats. The Flow Country is the largest blanket bog in Europe, and covers a large inland area in the west of the county. Caithness has a land boundary with the historic county of Sutherland to the west and is otherwise bounded by sea. The land boundary follows a watershed and is crossed by two roads (the A9 and the A836) and by one railway (the Far North Line). Across the Pentland Firth, ferries link Caithness with Orkney, and Caithness also has an airport at Wick. The Pentland Firth island of Stroma is within Caithness.

From the 9th century the Caithness area was ruled by the Jarl of Orkney, who at different times owed allegiance to both Norway and Scotland. Caithness subsequently became a separate provincial lordship from Orkney in the 14th century, being an earldom controlled by the Earl of Caithness. The name was also used for the Diocese of Caithness from the 12th century to the 17th century. The diocese was larger than the later county, also including Sutherland. A shire called Caithness covering the same area as the earldom was created in 1641, after a couple of earlier abortive attempts. Shires gradually eclipsed the old provinces in administrative importance, and also became known as counties.

The county ceased to be used for local government purposes in 1975, when the area became part of the Highland region, which in turn became a single-tier council area in 1996. There was a local government district called Caithness from 1975 to 1996, which was a lower-tier district within the Highland region. The pre-1975 county boundaries are still used for certain functions, being a registration county.

The Norn language was historically the language of everyday communication for people in Caithness, but was gradually overtaken by Scots and then English.

The Caith element of the name Caithness comes from the name of a Pictish tribe known as the Cat, Catt or Catti people, whose Kingdom of Cat covered what would become Caithness and parts of Sutherland from the 9th century. The -ness element comes from Old Norse and means "headland". The Norse called the area Katanes ("headland of the Catt people"), and over time this became Caithness.

The Gaelic name for Caithness, Gallaibh, means "among the strangers", referring to the Norse. The name of the Catti survives in the Gaelic name for eastern Sutherland, Cataibh, and in the old Gaelic name for Shetland, Innse Chat.

Caithness extends about 30 miles (48 km) north-south and about 30 miles (48 km) east-west, with a roughly triangular-shaped area of about 712 sq mi (1,840 km²). The topography is generally flat, in contrast to the majority of the remainder of the North of Scotland. Until the latter part of the 20th century when large areas were planted in conifers, this level profile was rendered still more striking by the almost total absence of woodland.