The Thames Estuary is a large estuary where the River Thames flows into the North Sea. The estuary, which begins at the Thames Barrier in London, is one of the largest inlets on the coast of Great Britain and is a major shipping route.

The term is also applied to the lands on either side of the estuary in East London and the counties of Kent and Essex. This areas has been designated as one of the principal development areas in Southern England by the Office of the Deputy Prime Minister. In this context, it is referred to as Thames Gateway.