Scapa Flow (; from Old Norse Skalpaflói 'bay of the long isthmus') is a body of water in the Orkney Islands, Scotland, sheltered by the islands of Mainland, Graemsay, Burray, South Ronaldsay and Hoy. Its sheltered waters have played an important role in travel, trade and conflict throughout the centuries. Vikings anchored their longships in Scapa Flow more than a thousand years ago. It was the United Kingdom's chief naval base during the First and Second World Wars, but the facility was closed in 1956.

Scapa Flow has a shallow sandy bottom not deeper than 60 metres (200 ft) and most of it is about 30 m (100 ft) deep; it is one of the great natural harbours and anchorages of the world, with sufficient space to hold a number of navies. The harbour has an area of 324.5 square kilometres (125.3 sq mi) and contains just under 1 billion cubic metres of water.

Since the scuttling of the German fleet after World War I, its wrecks and their marine habitats form an internationally acclaimed diving location.

Scapa Flow hosts an oil port, the Flotta Oil Terminal. In good weather, its roadstead (water of moderate conditions) allows ship-to-ship transfers of crude oil product. The world's first ship-to-ship transfer of liquefied natural gas (LNG) took place in Scapa Flow in 2007 transferring 132,000m³ of LNG. This occurred in 2007 by Excelerate Energy between the vessels Excalibur and Excelsior.