The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the north of the Arctic Circle, and south and nearby to the south of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous 24 hours, mostly north of the Arctic Circle and south of the Antarctic Circle. The number of days per year with potential midnight sun increases the farther poleward one goes from the equator. Although approximately defined by the polar circles, in practice midnight sun can be seen as much as 90 km beyond the polar circle, because the sun is a disk and not a point, and the exact latitudes of the farthest reaches of midnight sun depend on topography and vary slightly year-to-year.

There are no permanent human settlements south of the Antarctic Circle, so the countries and territories whose populations experience it are limited to the ones crossed by the Arctic Circle, e.g. Canada (Yukon, Northwest Territories, and Nunavut), Denmark (Greenland), Finland, Sápmi, Norway, Russia, Sweden, the United States (Alaska), and extremities of Iceland. A quarter of Finland's territory lies north of the Arctic Circle and at the country's northernmost point the sun does not set at all for 60 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles where the sun can be continuously visible for a half year.