Hardangervidda (English: Hardanger Plateau) is a mountain plateau (Norwegian: vidde) in central southern Norway, covering parts of Vestland, Telemark, and Buskerud counties. It is the largest plateau of its kind in Europe, with a cold year-round alpine climate, and one of Norway's largest glaciers, Hardangerjøkulen, is situated here. Much of the plateau is protected as part of Hardangervidda National Park. Hardangervidda is a popular tourist and leisure destination, and it is ideal for many outdoor activities.

The plateau is the largest peneplain (eroded plain) in Europe, covering an area of about 6,500 km² (2,500 sq mi) at an average elevation of 1,100 metres (3,600 ft). The highest point on the plateau is the Sandfloegga, which reaches a height of 1,721 m (5,646 ft).

The landscape of the Hardangervidda is characterised by barren, treeless moorland interrupted by numerous pools, lakes, rivers and streams. There are significant differences between the west side, which is dominated by rocky terrain and expanses of bare rock, and the east side, which is much flatter and more heavily vegetated. The climate also varies between the two sides: it is considerably wetter on the west side than on the east, with over 1,000 millimetres (39 in) per year recorded in some parts. The prominent peak of Hårteigen 1,690 m (5,545 ft) is visible across much of the plateau.

Much of the Hardangervidda's geology is extremely ancient. The rolling fells of the Hardangervidda are the remnants of mountains that were worn down by the action of glaciers during the ice ages. The bedrock is mainly of Precambrian and Cambro-Silurian origin.

The area of the Hardangervidda was once part of the Sub-Cambrian peneplain before it was thrust over by the nappes of the Caledonian orogeny in Paleozoic times. Much later, in the Miocene epoch, the modern flatness of the Hardangervidda took form as a peneplain formed at sea level. Then in Early Pliocene times the Hardangervidda and the whole of southern Scandinavian Mountains were uplifted more than a thousand meters.

The whole of the Hardangervidda is above the tree line. Its alpine climate enables the presence of many species of arctic animals and plants further south than anywhere else in Europe. Its wild reindeer herds are among the largest in the world, with some 15,000 animals recorded in 1996 and about 8,000 in 2008. They migrate across the plateau during the year, moving from their winter grazing lands on the east side of the Hardangervidda, where they graze on lichen, to their breeding grounds in the more fertile west of the plateau. On 26 August 2016, 323 were killed by lightning, prompting a Norwegian Environment Agency discussion on whether to leave so many corpses in the open.

The varying climate of the plateau has a marked effect on the flora, which is richer on the wetter west side than in the drier east; much of the plateau is covered by coarse grasses, mosses (especially sphagnum) and lichens.

Ciclosporin, an immunosuppressant drug widely used in organ transplantation to prevent rejection was initially isolated from the fungus Tolypocladium inflatum (Beauveria nivea), found in a soil sample obtained in 1969 from Hardangervidda.