In physical geography, a tundra (/ˈtʌndrə, ˈtʊn-/) is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. There are three regions and associated types of tundra: Arctic, Alpine, and Antarctic.

Tundra vegetation is composed of dwarf shrubs, sedges, grasses, mosses, and lichens. Scattered trees grow in some tundra regions. The ecotone (or ecological boundary region) between the tundra and the forest is known as the tree line or timberline. The tundra soil is rich in nitrogen and phosphorus. The soil also contains large amounts of biomass and decomposed biomass that has been stored as methane and carbon dioxide in the permafrost, making the tundra soil a carbon sink. As global warming heats the ecosystem and causes soil thawing, the permafrost carbon cycle accelerates and releases much of these soil-contained greenhouse gases into the atmosphere, creating a feedback cycle that contributes to climate change.

The word comes from the Russian “ту́ндра” (tundra). The first use of tundra in English was in 1824, spelled "toundra", possibly indicating borrowing from French. The origin of the Russian word is uncertain: some sources attribute the origin to "tunturia", Finnish for "treeless plain". Other sources say it originated from Sámi languages as "tundar", meaning "elevated wasteland" or "marshy plain", from the 16th century.

Arctic tundra occurs in the far Northern Hemisphere (Arctic), north of the taiga belt. The word "tundra" usually refers only to the areas where the subsoil is permafrost, or permanently frozen soil. (It may also refer to the treeless plain in general so that northern Sápmi would be included.) Permafrost tundra includes vast areas of northern Russia and Canada. The polar tundra is home to several peoples who are mostly nomadic reindeer herders, such as the Nganasan and Nenets in the permafrost area (and the Sámi in Sápmi).

Arctic tundra contains areas of stark landscape and is frozen for much of the year. The soil there is frozen from 25 to 90 cm (10 to 35 in) down, making it impossible for trees to grow there. Instead, bare and sometimes rocky land can only support certain kinds of Arctic vegetation, low-growing plants such as moss, heath (Ericaceae varieties such as crowberry and black bearberry), and lichen.

There are two main seasons, winter and summer, in the polar tundra areas. During the winter it is very cold, dark, and windy with the average temperature around −28 °C (−18 °F), sometimes dipping as low as −50 °C (−58 °F). However, extreme cold temperatures on the tundra generally do not drop as low as those experienced in taiga areas further south (for example, Russia's, Canada's, and Alaska's lowest temperatures were recorded in locations south of the tree line). During the summer, temperatures rise somewhat, and the top layer of seasonally-frozen soil melts, leaving the ground very soggy. The tundra is covered in marshes, lakes, bogs, and streams during the warm months. Generally daytime temperatures during the summer rise to about 12 °C (54 °F) but can often drop to 3 °C (37 °F) or even below freezing. Arctic tundras are sometimes the subject of habitat conservation programs. In Canada and Russia, many of these areas are protected through a national biodiversity action plan.

Arctic tundra tends to be windy, with winds often blowing upwards of 50–100 km/h (31–62 mph). It is also a polar desert, with only about 150–250 mm (6–10 in) of precipitation falling per year (the summer is typically the season of maximum precipitation). Although precipitation is light, evaporation is also relatively minimal. During the summer, the permafrost thaws just enough to let plants grow and reproduce, but because the ground below this is frozen, the water cannot sink any lower, so the water forms the lakes and marshes found during the summer months. There is a natural pattern of accumulation of fuel and wildfire which varies depending on the nature of vegetation and terrain. Research in Alaska has shown fire-event return intervals (FRIs) that typically vary from 150 to 200 years, with dryer lowland areas burning more frequently than wetter highland areas.

The biodiversity of tundras is low: 1,700 species of vascular plants and only 48 species of land mammals can be found, although millions of birds migrate there each year for the marshes. There are also a few fish species. There are few species with large populations. Notable plants in the Arctic tundra include blueberry (Vaccinium uliginosum), crowberry (Empetrum nigrum), reindeer lichen (Cladonia rangiferina), lingonberry (Vaccinium vitis-idaea), and Labrador tea (Rhododendron groenlandicum). Notable animals include reindeer (caribou), musk ox, Arctic hare, Arctic fox, snowy owl, ptarmigan, northern red-backed voles, lemmings, the mosquito, and even polar bears near the ocean. The tundra is largely devoid of poikilotherms such as frogs or lizards.