The Southern Hemisphere is the half (hemisphere) of Earth that is south of the equator. It contains all or part of five continents (the whole of Antarctica, the whole of Australia, about 90% of South America, about one-third of Africa, and some islands off the continental mainland of Asia) and four oceans (the whole Southern Ocean, the majority of the Indian Ocean, the South Atlantic Ocean, and the South Pacific Ocean), as well as New Zealand and most of the Pacific Islands in Oceania. Its surface is 80.9% water, compared with 60.7% water in the Northern Hemisphere, and it contains 32.7% of Earth's land.

Owing to the tilt of Earth's rotation relative to the Sun and the ecliptic plane, summer is from December to February (inclusive) and winter is from June to August (inclusive). September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox. The South Pole is in the centre of the southern hemispherical region.

Southern Hemisphere climates tend to be slightly milder than those at similar latitudes in the Northern Hemisphere, except in the Antarctic which is colder than the Arctic. This is because the Southern Hemisphere has significantly more ocean and much less land; water heats up and cools down more slowly than land. The differences are also attributed to oceanic heat transfer and differing extents of greenhouse trapping.

In the Southern Hemisphere, the Sun passes from east to west through the north, although north of the Tropic of Capricorn the mean Sun can be directly overhead or due south at midday. The Sun follows a right-to-left trajectory through the northern sky unlike the left-to-right motion of the Sun when seen from the Northern Hemisphere as it passes through the southern sky. Sun-cast shadows turn anticlockwise throughout the day and sundials have the hours increasing in the anticlockwise direction. During solar eclipses viewed from a point to the south of the Tropic of Capricorn, the Moon moves from left to right on the disc of the Sun (see, for example, photos with timings of the solar eclipse of November 13, 2012), while viewed from a point to the north of the Tropic of Cancer (i.e., in the Northern Hemisphere), the Moon moves from right to left during solar eclipses.

The Coriolis effect causes cyclones and tropical storms to spin clockwise in the Southern Hemisphere, as opposed to anticlockwise in the Northern Hemisphere.

The southern temperate zone, a subsection of the Southern Hemisphere, is nearly all oceanic.

The Sagittarius constellation that includes the galactic centre is a southern constellation as well as both Magellanic Clouds. This, combined with clearer skies, makes for excellent viewing of the night sky from the Southern Hemisphere with brighter and more numerous stars.

Forests in the Southern Hemisphere have special features which set them apart from those in the Northern Hemisphere. Both Chile and Australia share, for example, unique beech species or Nothofagus, and New Zealand has members of the closely related genera Lophozonia and Fuscospora. The eucalyptus is native to Australia but is now also planted in Southern Africa and Latin America for pulp production, and increasingly, biofuel uses.