Physa is a genus of small, left-handed or sinistral, air-breathing freshwater snails, aquatic pulmonate gastropod molluscs in the subfamily Physinae of the family Physidae.

Members of the freshwater pulmonate family Physidae possess a complex of muscles that is unique amongst gastropods.

The physid musculature is responsible for a unique ability of physids to rapidly flick their shells from side to side — a reaction that frequently enables them to escape predation.

These small snails, like all the species in the family Physidae, have shells that are sinistral, which means that when the shell is held with the spire pointing up and the aperture facing the viewer, then the aperture is on the left-hand side.

The shells of Physa species have a long and large aperture, a pointed spire, and no operculum.