St Margaret's Island is a small island adjoining Caldey Island in Carmarthen Bay, Wales. Its name derives from a chapel that once stood on the site; however, in Victorian times this was converted into housing for local quarry workers, who mined limestone on the island until 1851.
St Margaret's Island is a sensitive conservation site because of the birds that nest in its cliffs: cormorants (the largest population of this species in Wales, constituting 3% of the total British population), guillemots, razorbills, shags, kittiwakes, great black-backed gulls, lesser black-backed gulls, herring gulls, puffins and, more recently, gannets. The island is vegetated with sea beet, sea mayweed and rank grass.
There is no public access to St Margaret's Island. However, the waters around it are popular with divers.