'Deadlight' was the British operation to scuttle U-boats surrendered by Germany at the end of World War II (17 November 1945/12 February 1946).

Of the 156 boats which surrendered, including one interned in Spain, 116 were scuttled in deep water off Lisahally in Northern Ireland or Loch Ryan in Scotland late in 1945 and early in 1946. At 10.04 on 12 February 1946 U-3514 became the last U-boat sunk in 'Deadlight'.

The operation was the responsibility of the Royal Navy, which planned to tow the boats to three areas about 100 miles (160 km) to the north-west of Ireland for sinking. The areas were codenamed XX, YY and ZZ: it was intended that XX would be used as the main area for scuttling, ZZ would be used for practice and experimental air attacks on 36 boats, and YY was to be a reserve position to which, the weather permitting, boats could be diverted from XX for practice and experimental destruction by naval forces. In the case of those submarines not reserved for use as targets, the plan was to sink them with explosive charges, although naval gunfire was a fall-back option if that failed.

When the operation was started, it was discovered that many of the boats were in an extremely poor condition as they had been mored without protection in exposed harbours while awaiting disposal. In combination with adverse winter weather conditions, this meant that 56 of the boats sank before reaching the designated scuttling areas, and that those which did reach the disposal area were usually sunk by gunfire rather than explosive charges.

The first batch of 11 boats was sunk on 27/28 November 1945, and the last boat was dispatched on 12 February 1946.

The 33 surviving boats were used for various experimental and developmental tasks and then sunk: U-1105, for example, was sunk in an explosives trial in the Potomac river estuary in 1948. Of the several U-boats which survived 'Deadlight', some were claimed as prizes by Norway, the UK and the USSR. Five were in the Far East when Germany surrendered in May 1945, and were then commandeered by Japan.