沙丘，是指一种在风力作用下沙粒堆积的地貌。其外表呈丘状或垄状堆积，一般高度可达几米甚至几十米，个别有超过百米者。沙丘的存在是风力将未固结的物质吹移堆积而造成的。沙丘在荒漠或半荒漠地带分布最广，在海岸、湖滨以至于河岸的沙滩也可以有分布。

沙丘按流动程度分为固定沙丘、半固定沙丘和流动沙丘，形态上有新月形、金字踏形、蜂窝形、格状、抛物线形等各种形状。裸露的沙丘容易流动，可以淹没耕地、房屋、道路等，生长植物的沙丘比较固定。

沙丘的沙由于经过风力的长期选择，粒度均一，含泥量很少，适合作为各种建筑材料和工业原料。

In physical geography, a dune is a hill of sand built by either wind or water flow. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes.