Mammatus (also known as mamma or mammatocumulus, meaning "breast-cloud") is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud, often a cumulus or cumulonimbus. Mammatus usually appear as a storm is approaching or leaving.

While most clouds form in rising air, mammatus form in sinking air. Updrafts in a storm carry air that is saturated with precipitation to the cloud tops, where it loses upward momentum and eventually sinks back toward the ground. Although the air warms as it sinks, most of its heat energy goes toward melting and evaporating the precipitation particles contained within it. As the sinking air becomes cooler than its surroundings, it extends below the cloud base in the form of sagging mammatus.

Their colour is normally the same as that of the host cloud (white or blue-grey), but direct illumination from the setting sun and other clouds may cause a gold or reddish cast.