Western Australia (commonly abbreviated as WA) is a state of Australia, occupying the entire western third of the Australian continent. It is bordered by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east. Western Australia is Australia's largest state and the second largest subnational entity in the world. It has 2.2 million inhabitants (10% of the national total), 85% of whom live in the south-west corner of the state.

The state's capital city is Perth. The people of Western Australia are sometimes referred to as sandgropers, the common name of an insect found on sand dunes around Perth.

The first European to visit Western Australia was the Dutch explorer Dirk Hartog, who visited the Western Australian coast in 1616. The British established a military outpost at King George Sound, near present-day Albany, in 1826, which was followed by the establishment of the Swan River Colony in 1829, including the townsite of the present-day capital, Perth. Western Australia achieved self-government in 1890, and later federated with the other British colonies in Australia in 1901. Today Western Australia's economy mainly relies on mining, agriculture and tourism. WA supplies 36% of Australia's overseas exports and is the third largest iron-ore producer in the world.

Western Australia is bounded by South Australia and the Northern Territory to the east, and the Indian Ocean to the west and north. The International Hydrographic Organization (IHO) designates the body of water south of the continent as part of the Indian Ocean, although they are officially gazetted as the Southern Ocean in Australia.

The total distance of the state's eastern borders is 1,862 km (1,157 mi), and there is 12,889 km (8,009 mi) of coastline. The total land area occupied by the state is 2.5 million km2.

The southwest coastal area has a Mediterranean climate and was originally heavily forested, including large stands of the karri, one of the tallest trees in the world. This agricultural region of Western Australia is in the top nine terrestrial habitats for terrestrial biodiversity, with a higher proportion of endemic species than most other equivalent regions. Thanks to the offshore Leeuwin Current the area numbers in the top six regions for marine biodiversity, containing the most southerly coral reefs in the world.

Average annual rainfall varies from 300 millimetres (12 in) at the edge of the Wheatbelt region to 1,400 millimetres (55 in) in the wettest areas near Northcliffe, but in the months of November to March evaporation exceeds rainfall, and it is generally very dry. Plants must be adapted to this as well as the extreme poverty of all soils. A major reduction in winter rainfall has been observed since the mid-1970s, with a greater number of extreme rainfall events in the summer months.

The central four-fifths of the state is semiarid or desert and is lightly inhabited with the only significant activity being mining. Annual rainfall averages 200–250 millimetres (8–10 in), most of which occurs in sporadic torrential falls related to cyclone events in summer months.

An exception to this is the northern tropical regions. The Kimberley has an extremely hot monsoonal climate with average annual rainfall ranging from 500 to 1,500 millimetres (20–60 in), but there is a very long almost rainless season from April to November. Eighty-five percent of the state's runoff occurs in the Kimberley, but because it occurs in violent floods and because of the insurmountable poverty of the generally shallow soils, the only development has taken place along the Ord River.

Occurrence of snow in the state is rare, and typically only in the Stirling Range near Albany, as it is the only mountain range far enough south and with sufficient elevation. More rarely, snow can fall on the nearby Porongurup Range. Snow outside these areas is a major event; it usually occurs in hilly areas of southwestern Australia. The most widespread low-level snow occurred on 26 June 1956 when snow was reported in the Perth Hills, as far north as Wongan Hills and as far east as Salmon Gums. However, even in the Stirling Range, snowfalls rarely exceed 5 cm (2 in) and rarely settle for more than one day.

The highest observed maximum temperature of 50.5 °C (122.9 °F) was recorded at Mardie, Pilbara, 61.6 kilometres (38.3 mi) from Barrow Island on 19 February 1998. The lowest minimum temperature recorded was -7.2 °C (19.0 °F) at Eyre Bird Observatory on 17 August 2008.