The Federation of Australia was the process by which the six separate British self-governing colonies of Queensland, New South Wales, Victoria, Tasmania, South Australia (which also governed what is now the Northern Territory), and Western Australia agreed to unite and form the Commonwealth of Australia, establishing a system of federalism in Australia. The colonies of Fiji and New Zealand were originally part of this process, but they decided not to join the federation. Following federation, the six colonies that united to form the Commonwealth of Australia as states kept the systems of government (and the bicameral legislatures) that they had developed as separate colonies, but they also agreed to have a federal government that was responsible for matters concerning the whole nation. When the Constitution of Australia came into force, on 1 January 1901, the colonies collectively became states of the Commonwealth of Australia.

The efforts to bring about federation in the mid-19th century were dogged by the lack of popular support for the movement. A number of conventions were held during the 1890s to develop a constitution for the Commonwealth. Sir Henry Parkes, Premier of the Colony of New South Wales, was instrumental in this process. Sir Edmund Barton, second only to Parkes in the length of his commitment to the federation cause, was the caretaker Prime Minister of Australia at the inaugural national election in March 1901. The election returned Barton as prime minister, though without a majority.

This period has lent its name to an architectural style prevalent in Australia at that time, known as Federation architecture, or Federation style.