The Antarctic Circle is one of the five major circles (or parallels) of latitude that mark maps of the Earth. As of 2009, it lies at latitude 66° 33′ 39″ (or 66.56083°) south of the equator. The area south of the Antarctic Circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone.

The equivalent line of latitude in the northern hemisphere is the Arctic Circle. Every place south of the Antarctic Circle experiences a period of twenty-four hours' continuous daylight at least once per year, and a period of twenty-four hours' continuous night time at least once per year. That is to say, there is at least one whole day during which the sun does not set, and at least one whole day during which the sun does not rise.

On the Antarctic Circle these events occur, in principle, exactly once per year, at the December solstice and the June solstice respectively. This happens because the Earth's axis is tilted, by approximately 23.5 degrees, relative to ecliptic (the plane of the Earth's orbit around the sun). At the June solstice the southern hemisphere is tilted away from the Sun to its maximum extent, and the region of permanent darkness reaches its northern limit; at the December solstice the southern hemisphere is tilted towards the Sun to its maximum extent, and the region of permanent sunlight reaches its northern limit.

In practice several other factors affect the appearance of continuous day or night, the most important being atmospheric refraction, the altitude of the observer above sea level, mirages, and the fact that the sun is a sphere rather than a point. Mirages on the Antarctic continent tend to be even more spectacular than in Arctic regions, creating, for example, a series of apparent sunsets and sunrises while in reality the sun remains under the horizon. Due to gradual changes in the tilt of the Earth's axis, the Antarctic Circle is slowly moving.

See circles of latitude.

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