Lacuna is a genus of sea snails, marine gastropod mollusks in the family Littorinidae, the winkles or periwinkles.

There are two slender, contractile tentacles with the eyes on small pedicels near their base.

The thin shell has an ovate-conical or subglobose shape and is covered with a delicate horny periostracum.

The oval or roundish aperture is covered with a horny, spirally marked operculum.

The columella is flattened and forms with the peristome an elongated groove continued from the umbilicus.