Leigh-on-Sea is a town in southeast Essex, England. Since 1913, it has been administered as part of the Borough of Southend-on-Sea, (the centre of which is located three miles to the east), but has retained a distinct identity. In 1996, it became a civil parish, and a town council was created, funded by an increased council tax rate in the area. The parish, which is the only in Southend, had a population of 20,737 according to the 2001 census.

Leigh-on-Sea was recorded in the Domesday Book of 1086 as Legra. Located next to the sea, Leigh has been primarily a fishing village for most of its history. However, its sheltered position at the mouth of the Thames gave it some success as a port, with international trade and a shipbuilding business. The Mayflower may have been built in Leigh-on-Sea; it was certainly fitted out there.

The Mayflower was the ship which transported the Pilgrims from Plymouth, England to "North Virginia" (in what was later to become the United States of America) in 1620, leaving Plymouth on September 6 and dropping anchor near Cape Cod on November 11 (both dates according to the Old Style or the Julian Calendar). This voyage followed and was inspired by the successful establishment of the first permanent English settlement, Jamestown, by the London Company of Virginia in 1607.

The arrival of the London, Tilbury and Southend Railway in 1851 spurred the town's development, allowing greater trade with London and the rest of the world, and making it a commuter town for London workers.

It has a thriving artistic community, and holds an annual Art Trail where local and invited international artists show their works in the area's shops and cafes.

Leigh Regatta is held over one weekend in September and draws crowds from around Essex to its celebration of seafood and maritime games, such as climbing the greasy pole and tug-of-war. It is organised by the three Sea Scout Groups based in the Old Town to raise funds for local Scouting and a nominated charity.