A cairn is a man-made pile (or stack) of stones raised for a purpose, usually as a marker or as a burial mound. The word cairn comes from the Scottish Gaelic: càrn [ˈkʰaːrˠn̪ˠ] (plural càirn [ˈkʰaːrˠɲ]).

Cairns have been and are used for a broad variety of purposes. In prehistoric times, they were raised as markers, as memorials and as burial monuments (some of which contained chambers). In modern times, cairns are often raised as landmarks, especially to mark the summits of mountains. Cairns are also used as trail markers. They vary in size from small stone markers to entire artificial hills, and in complexity from loose conical rock piles to elaborate megalithic structures. Cairns may be painted or otherwise decorated, whether for increased visibility or for religious reasons.

A variant is the inuksuk (plural inuksuit), used by the Inuit and other peoples of the Arctic region of North America.