CANZUK is a proposed alliance of Canada, Australia, New Zealand and the United Kingdom to form an international organisation or confederation similar in scope to the former European Economic Community. This includes increased trade, foreign policy co-operation, military co-operation and mobility of citizens between the four states. This is tied together by similar economic systems, social values and political and legal system, a shared head of state being King Charles III, in addition to the majority population of each country speaking English. The idea is lobbied by the advocacy group CANZUK International. Other supporters include think tanks such as the Adam Smith Institute, the Henry Jackson Society, Bruges Group and politicians from the four countries.

The term CANZUK was first coined by William David McIntyre in his 1967 book Colonies into Commonwealth in the context of a "CANZUK Union". The idea of increased migration, trade and foreign policy cooperation between the CANZUK countries was created and popularized in 2015 by CEO and Founder of CANZUK International (formerly the Commonwealth Freedom of Movement Organisation), James Skinner.

In the wake of the 2016 United Kingdom European Union membership referendum and the decision made by Britain to leave the European Union, writers such as Andrew Lilico and James C. Bennett, along with academics such as the historian Andrew Roberts, also advocated that Canada, Australia, New Zealand and the United Kingdom merge and form a new entity in international politics. Roberts suggested that such a bloc could slot into the international order as a third pillar of the West (alongside the United States and the European Union). Beyond this, Roberts argues that due to its territorial scale, geographic scope and advanced economy that it would qualify as a "great power" and potentially a "global power" (or emerging superpower).

Some advocates such as Roberts favour a federal or confederal union. Others, such as Lilico, describe the objective as being the creation of a "geopolitical partnership" akin to the European Economic Community. In the version favoured by Lilico, by the advocacy group CANZUK international and by the Conservative Party of Canada, the proposal would involve the creation of a free-movement zone, a multilateral free trade agreement and a security partnership. The more general concept of deepening trade ties (with or without a multilateral agreement) has many advocates, including figures such as former Australian Prime Minister Scott Morrison, former Canadian Prime Minister Justin Trudeau, former British Prime Minister Theresa May and former New Zealand Prime Minister Jacinda Ardern.

Canada, Australia and New Zealand are former dominions of the British Empire where people of British ethnic origin came to constitute the majority of the population. Today, the four CANZUK countries maintain a close affinity of cultural, diplomatic and military ties to one another. The Australian and New Zealand national flags contain the Union Flag in their canton, and the Union Flag is a ceremonial flag in Canada (referred to as the Royal Union Flag).

Canada, Australia, New Zealand and the United Kingdom are also Commonwealth realms which share Charles III as their constitutional monarch and their head of state. The countries share a number of institutional, linguistic and religious similarities such as the use of political systems based upon the Westminster parliamentary system of government, and common law. The CANZUK countries form part of the English-speaking world and share a number of Anglosphere military initiatives with each other including the Fincastle Trophy, Five Eyes intelligence, ABCANZ Armies and AUSCANNZUKUS, which are concerned with increased military and naval co-operation. Canada and the United Kingdom are allied through the North Atlantic Treaty Organization while Australia, New Zealand and the United Kingdom are allied through the Five Power Defence Arrangements.

All four nations have diverse, multicultural populations, free and open presses, and are closely aligned on key social issues. Public relations are extremely warm between the four countries, with consistent evidence that people in Canada, Australia, New Zealand and the United Kingdom regard each other's countries as their own country's closest friends and allies in the world, including the countries joining the United States of America in the Five Eyes intelligence sharing community.

Since 1983, Australia and New Zealand have had formal trade ties with the Closer Economic Relations (CER) agreement.