The Imperial Federation was a series of proposals in the late 19th and early 20th centuries to create a federal union to replace the existing British Empire, presenting it as an alternative to colonial imperialism. No such proposal was ever adopted, but various schemes were popular in Australia, Canada, New Zealand, and other colonial territories. The project was championed by Unionists such as Joseph Chamberlain as an alternative to William Gladstone's proposals for home rule in Ireland.

Many proposals were put forward, but none commanded majority support. The Imperial Federation League, the main advocacy group, split into two factions in 1893, with one group promoting imperial defence and the other encouraging imperial trade. Various proposals were put forward, with most of them calling for a single state with an imperial parliament headquartered in London. Such proposals were never put into effect, and decolonisation would eventually happen to a vast majority of Britain's colonies beginning from the mid 20th century.

The new parliament was envisioned to cooperatively deal with internal trade, foreign relations, defence, and other issues that affected the entire federation. The new parliament would have representatives from India that would rule the latter directly, while the dominions such as Australia, Canada, New Zealand, Newfoundland, and South Africa as well as crown colonies such as Cyprus, Gibraltar, Malta, and Singapore would have internal self-government, although still accountable to this new parliament in London, similar to devolution that was granted to Northern Ireland, Scotland, and Wales during the late 20th century. In the Imperial Federation, Ireland would also have self-government, which was expected to reduce demand for independence in Ireland.

By the 1880s the British Empire covered a quarter of the world's land area, and included a fifth of the world's population. There was no doubt about the vastness of the potential, and there was agreement that opportunities were largely wasted because politically and constitutionally there was no unity, no common policies, no agreed central direction, no "permanent binding force" said Alfred Milner. Associations were formed and discussions were held to come up with a solution.

The British Empire consisted of many colonies, several of which were largely self-governing dominions (Canada, Newfoundland, Australia, New Zealand, South Africa, and the Straits Settlements). Most were ruled by colonial officials including India, the West Indies, and Fiji. The future of the empire remained uncertain, as it was unclear what the result would be if all colonies eventually became self-governing. Among other concerns, it would be very difficult for British interests to be maintained if every colony was essentially already sovereign.

New national identities emerging throughout the dominions was also a cause for concern. Growing autonomy for Canada, Australia, New Zealand, and South Africa led to each of them adopting coats of arms of their own rather than continuing to use those of the United Kingdom. Some felt that the growing status of the dominions was not reflected in British symbols such as the Union Jack, and this led to calls for the creation of unifying symbols such as a British Empire flag to represent a new multinational reality.

Creating an Imperial Federation thus became a popular alternative proposal to colonial imperialism. The plan was never firm, but the general proposal was to create a single federal state among all colonies of the British Empire. The federation would have a common parliament and would be governed as a superstate. Thus, Imperial unity could be maintained while still allowing for democratic government. The colonies would increase their influence while Britain would be able to share the costs of imperial defence. The best features of large states could be combined with the best features of small states. It was seen as a method of solving the Home Rule problem in Ireland, as England, Scotland, Wales, and Ireland (along with the other members of the Old Commonwealth) would have their own Parliaments. Westminster would become a purely Imperial body.

Supporters of the Imperial Federation regarded the United Kingdom as having two possible futures: imperial union of its colonies consisting of different ethnic groups and continued long-term importance in global affairs, or imperial dissolution and the reduction of the status of the country to a second-class nation with little to no power projection. In response to claims that geography was against federation on such a large scale, it was said that scientific advancements would solve the difficulty. Edward Ellis Morris reminded listeners to his lecture in 1885 that it was now as easy to reach London from Melbourne or Singapore as it had been to reach London from Orkney at the time of the Acts of Union 1707, or to reach Washington, D.C. from San Francisco before 1869.