A grotto (Italian grotta and French "grotte") is any type of natural or artificial cave that is associated with modern, historic or prehistoric use by humans. When it is not an artificial garden feature, a grotto is often a small cave near water and often flooded or liable to flood at high tide. The picturesque Grotta Azzurra at Capri and the grotto of the villa of Tiberius in the Bay of Naples are outstanding natural seashore grottoes. Whether in tidal water or high up in hills, they are very often in limestone geology where the acidity dissolved in percolating water has dissolved the carbonates of the rock matrix as it has passed through what were originally small fissures. See karst topography, cavern.

Mantic springs that issued from grottoes were a feature of Apollo's oracles at Delphi, Corinth, and Clarus. The new-built Hellenistic city of Rhodes was provided with rock-cut artificial grottos with "naturalistic" features. At the great Roman sanctuary of Praeneste south of Rome, the oldest portion of the primitive sanctuary was situated on the next-to-lowest terrace, in a grotto in the natural rock where there was a spring that developed into a well. Such a sacred spring had its native nymph, who might be honored in a grotto-like nymphaeum, where the watery element was never far to seek.