A Commonwealth realm is a sovereign state in the Commonwealth of Nations whose monarch and head of state is shared among the other realms. Each realm is an independent state, equal with the other realms and nations of the Commonwealth. King Charles III succeeded his mother, Queen Elizabeth II, as monarch of each Commonwealth realm following her death on 8 September 2022. He simultaneously became Head of the Commonwealth.

As of 2023 there are 15 Commonwealth realms: Antigua and Barbuda, Australia, The Bahamas, Belize, Canada, Grenada, Jamaica, New Zealand, Papua New Guinea, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Solomon Islands, Tuvalu and the United Kingdom. All are members of the Commonwealth, an intergovernmental organisation of 56 independent member states, 52 of which were formerly part of the British Empire. All Commonwealth members are independent sovereign states, regardless of whether they are Commonwealth realms.

At her accession in 1952, Queen Elizabeth II was the monarch and head of state of seven independent states: the United Kingdom, Canada, Australia, New Zealand, South Africa, Pakistan and Ceylon (now Sri Lanka). Since then, new realms have been created through the independence of former colonies and dependencies, and some realms have become republics. As of 2023 Barbados was the most recent realm to become a republic; it did so on 30 November 2021.