Flagstone (flag) is a generic flat stone, usually used for paving slabs or walkways, patios, fences and roofing. It may be used for memorials, headstones, facades and other constructions. The name derives from Middle English flagge meaning turf, perhaps from Old Norse flaga meaning slab or chip.

Flagstone is a sedimentary rock that is split into layers along bedding planes. Flagstone is usually a form of a sandstone composed of feldspar and quartz and is arenaceous in grain size (0.16 mm – 2 mm in diameter). The material that binds flagstone is usually composed of silica, calcite, or iron oxide. The rock color usually comes from these cementing materials. Typical flagstone colors are red, blue, and buff, though exotic colors exist.

Flagstone is quarried in places with bedded sedimentary rocks with fissile bedding planes. Examples include Arizona flagstone and Pennsylvania Bluestone.

Around the thirteenth century, the ceilings, walls and floors in European architecture became more ornate. Anglo-Saxons in particular used flagstones as flooring materials in the interior rooms of castles and other structures.Lindisfarne Castle in England and Muchalls Castle (14th century) in Scotland are among many examples of buildings with surviving flagstone floors.