Calshot Spit is a one mile long, sand and shingle bank located on the southern bank of the open end of Southampton Water on the south coast of England.

At the end of the spit are a castle, built by King Henry VIII, an RNLI lifeboat, a HM Coastguard station, several marine slips and a former Royal Navy and Royal Air Force flying boat station.

The castle is a low, circular, stone structure and was armed with large muzzle_loading guns. Its purpose when it was built in 1539 was to protect access to the important harbour at Southampton from seaborne attack by France or Spain from the English Channel via the Solent.

Calshot Naval Air Station was originally set up in 1913. Its choice as a location for operating flying boats is clear: the landing area is sheltered by land on the three sides of Southampton Water and by the Isle of Wight, several miles away, on the fourth side. By the end of World War I flying boat patrols from Calshot had sunk three U-boats.

Between the wars Calshot was transferred to the Royal Air Force. The spit was the venue of the Schneider Trophy races of 1929 and 1931. Aircraftsman Shaw, also known as Lawrence of Arabia, worked there around that time.

During World War II, Calshot operated as a Short Sunderland repair base. The flying boat base closed in 1961. Two unfinished Saunders_Roe Princess flying boats were left at the Spit during the 1950s and 1960s.

The site is now used as a base for the RNLI, yachts and HM Coastguard. The hangers house indoor climbing walls, dry-ski slopes and a cycle track.