St Catherine's Island is a small tidal island linked to Tenby in Pembrokeshire, Wales, by Castle beach at low tide. The island, which is known colloquially as St Catherine's Rock, is the location of St Catherine's Fort.

A tidal island is a raised area of land within a waterbody, which is connected to the larger mainland by a natural isthmus or man-made causeway that is exposed at low tide and submerged at high tide, causing the land to switch between being a promontory/peninsula and an island depending on tidal conditions.