Chadwell St Mary is an area of the unitary authority of Thurrock in Essex, England. It is one of the traditional (Church of England) parishes in Thurrock and a former civil parish. Grays is 2 miles (3.2 km) to the west and 1 mile (1.6 km) to the south is Tilbury. The area is sometimes referred to simply as Chadwell, particularly before the 19th century.

The southern part of the parish was originally a natural salt marsh adjacent to the River Thames which is tidal and saline at this point. Decaying vegetation together with mud and silt from the river were deposited from the end of the most recent ice age until the marshes were reclaimed sometime before the 14th century. These sedimentary deposits reach a depth of fifty feet or more and are interleaved with a number of peaty layers.