Langstone Harbour is a 2,085.4-hectare (5,153-acre) biological Site of Special Scientific Interest in Hampshire. It is an inlet of the English Channel in Hampshire, sandwiched between Portsea Island to the south and west, Hayling Island to the south and east, and Langstone to the north. It is part of the Ramsar site of Langstone and Chichester Harbour Special Protection Area and Nature Conservation Review site. Parts are Special Areas of Conservation, or Local Nature Reserves, and some areas by the Hampshire and Isle of Wight Wildlife Trust.

Historically, the harbour has been used for salt and sand production, as well as fishing and latterly leisure boating. A ferry across the mouth of the harbour has been in operation for more than 200 years.

Langstone Harbour is an inlet of the English Channel in Hampshire, sandwiched between Portsea Island to the south and west, Hayling Island to the south and east, and Langstone to the north. It is part of the Ramsar site of Langstone and Chichester Harbour, Special Protection Area and Nature Conservation Review site, Grade I. Parts of it are in Solent Maritime and Solent and Isle of Wight Lagoons Special Areas of Conservation. Three areas are Local Nature Reserves, Farlington Marshes West Hayling and The Kench, Hayling Island. Two areas are nature reserves managed by the Hampshire and Isle of Wight Wildlife Trust, Farlington Marshes and Southmoor Nature Reserve

West of Portsmouth is Portsmouth Harbour and the three linked harbours are important recreational and conservation areas as well as supporting commercial fishing and shipping. It is administered by the Langstone Harbour Board. The eastern boundary with Chichester Harbour is defined by a historic causeway known as the Wade Way, which was originally the only crossing between Hayling Island and the mainland. It is now impassable, having been cut in two by a deep channel for the Portsmouth and Arundel Canal in the 1820s (for further information see Langstone).

Langstone Harbour contains a number of islands: Baker's Island (SU694035), North Binness Island (SU692046), South Binness Island (SU698031) and Long Island (SU701041). These are subject to erosion and during the 1990s a seven-year archaeology project took place before their history was lost to the seas.

There are two smaller islands: Round Nap Island, which is connected to South Binness Island by a tidal causeway; and Oyster Island, which contains the remains of an old hut, thought to have been built in 1819 to guard the oyster beds.

Langstone Harbour was originally a river valley of one of the tributaries flowing into the then River Solent. With the end of the last ice age sea levels rose until sometime between 4000 and 3500BC the harbour took on the form it would have until the 18th century.

For much of its history the harbour has been an area of salt production. The Domesday Book records three salthouses around the harbour and by the early 17th century a saltern at Copnor was well established. Here a large shallow area of the harbour meant that even without further improvement salt could be extracted from the area after each tide. The Copnor saltern ceased production in 1800 but salt production continued elsewhere in the harbour until 1933.