Coordinates: 51°47′45″N 0°56′24″E / 51.79570°N 0.94°E / 51.79570; 0.94

Mersea Island (formerly Mersey Island) is the most easterly inhabited island in the United Kingdom, located marginally off the coast of Essex, England, 9 miles (14 km) to the southeast of Colchester. It is situated in the estuary area of the Blackwater and Colne rivers and has an area of around 7 square miles (18 km²). The name 'Mersea' is derived from the Old English meresig meaning 'island of the pool'.

The island hosts a population of 6,925 and is the most easterly inhabited island in the UK. It is joined to the mainland by a causeway, known as The Strood. This carries the Mersea–Colchester road (B1025) which is often covered at high tides and especially including spring tides. It is close to neighbouring Ray Island. The water separating the island is the Pyefleet Channel to the east and the Strood Channel to the west. There are two main settlements on the island, the small town of West Mersea and the village of East Mersea, plus a small hamlet at Barrow Hill to the north of West Mersea.