沙蚕（Nereis），俗称沙虫，是多毛纲（Polychaeta）叶须虫目沙蚕科的一种环节动物。常栖息于泥沙中，生殖季节或夜出觅食时能游泳。可作为鱼类、虾、蟹等动物的食饵。

体长10公分左右。体形修长，呈圆柱形，两侧对称、后端尖。身体分节明显，体节两侧突出成具有刚毛的疣足用以行动。

Nereis is a genus of polychaete worms in the family Nereidae. It comprises many species, most of which are marine, including the sandworm (Nereis virens) and the common clam worm (Nereis succinea). Nereis possess setae and parapodia for locomotion. They may have two types of setae, which are found on the parapodia. Acicular setae provide support. Locomotor chaetae are for crawling, and are the bristles that are visible on the exterior of the Polychaeta.