The Colonial Service, also known as His/Her Majesty's Colonial Service and replaced in 1954 by Her Majesty's Overseas Civil Service (HMOCS), was the British government service that administered most of Britain's overseas possessions, under the authority of the Secretary of State for the Colonies and the Colonial Office in London. It did not operate in British India, where the same function was delivered by the Indian Civil Service, nor in the Anglo-Egyptian Sudan, which was administered by the Sudan Political Service, nor in the internally self-governing colony of Southern Rhodesia.

The Colonial Office was a government department of the Kingdom of Great Britain and later of the United Kingdom, first created in 1768 from the Southern Department to deal with colonial affairs in North America, until merged into the new Home Office in 1782. In 1801, colonial affairs were transferred to the War Office in the lead up to the Napoleonic Wars, which became the War and Colonial Office to oversee and protect the colonies of the British Empire. The Colonial Office was re-created as a separate department 1854, under the colonial secretary. It was finally merged into the Commonwealth Office in 1966.