Tartan (Scottish Gaelic: breacan [ˈpɾʲɛxkən]) is a patterned cloth consisting of criss-crossed, horizontal and vertical bands in multiple colours. Tartans originated in woven wool, but now they are made in other materials. Tartan is particularly associated with Scotland, as Scottish kilts almost always have tartan patterns.

Tartan is made with alternating bands of coloured (pre-dyed) threads woven as both warp and weft at right angles to each other. The weft is woven in a simple twill, two over—two under the warp, advancing one thread at each pass. This pattern forms visible diagonal lines where different colours cross, which give the appearance of new colours blended from the original ones. The resulting blocks of colour repeat vertically and horizontally in a distinctive pattern of squares and lines known as a sett. Outside of Scotland, tartan is also known as "plaid" (particularly in North America), however in Scotland, a plaid is a large piece of tartan cloth, worn as a type of kilt or large shawl.

The Dress Act of 1746 attempted to bring the warrior clans under government control by banning the tartan and other aspects of Gaelic culture. When the law was repealed in 1782, it was no longer ordinary Highland dress, but was adopted instead as the symbolic national dress of Scotland, a status that was widely popularised after King George IV wore a tartan kilt in his 1822 visit to Scotland. Until the middle of the nineteenth century, the highland tartans were only associated with either regions or districts, rather than any specific Scottish clan. This was because like other materials, tartan designs were produced by local weavers for local tastes and would usually only use the natural dyes available in that area, as synthetic dye production was non-existent and transportation of other dye materials across long distances was prohibitively expensive. The patterns were simply different regional checked-cloth patterns, chosen by the wearer's preference—in the same way as people nowadays choose what colours and patterns they like in their clothing, without particular reference to propriety. It was not until the mid-nineteenth century that patterns were created and artificially associated with Scottish clans, families, or institutions who were (or wished to be seen as) associated in some way with a Scottish heritage. The Victorians' penchant for ordered taxonomy and the new chemical dyes then available meant that the idea of specific patterns of bright colours, or "dress" tartans, could be created and applied to a nostalgic view of Scottish history. The Irish also wore tartan clothing but to a far lesser degree than their Gaelic cousins in Scotland.

Today tartan is no longer limited to textiles, but is also used as a name for the pattern itself, appearing on media such as paper, plastics, packaging, and wall coverings. The use of tartan has spread outside the British Isles, particularly to countries who have been influenced by Scottish culture.