The 1890 British Ultimatum was an ultimatum by the British government delivered on 11 January 1890 to the Kingdom of Portugal. The ultimatum forced the retreat of Portuguese military forces from areas which had been claimed by Portugal on the basis of historical discovery and recent exploration, but which the United Kingdom claimed on the basis of effective occupation. Portugal had attempted to claim a large area of land between its colonies of Mozambique and Angola including most of present-day Zimbabwe and Zambia and a large part of Malawi, which had been included in Portugal's "Rose-coloured Map".

It has sometimes been claimed that the British government's objections arose because the Portuguese claims clashed with its aspirations to create a Cape to Cairo Railway, linking its colonies from the south of Africa to those in the north. This seems unlikely, as in 1890 Germany already controlled German East Africa, now Tanzania, and Sudan was independent under Muhammad Ahmad. Rather, the British government was pressed into taking action by Cecil Rhodes, whose British South Africa Company was founded in 1888 south of the Zambezi and the African Lakes Company and British missionaries to the north.