A  barchans (Russian) or barkhan (Kazakh) is a crescent-shaped dune. Barchans face the wind, appearing convex and are produced by wind action predominately from one direction.

They are a very common landform in sandy deserts all over the world and are arc-shaped, markedly asymmetrical in cross section, with a gentle slope facing toward the wind sand ridge, comprising well-sorted sand.

Barchans usually occur as groups of isolated dunes and may form chains that extend across a plain in the direction of the prevailing wind. Barchans and megabarchans may coalesce into ridges that extend for hundreds of kilometres.

Barchan dunes have been observed on Mars, where the thin atmosphere produces winds strong enough to move sand and dust.