The Kingdom of Romania was a constitutional monarchy that existed from 13 March (O.S.) / 25 March 1881 with the crowning of prince Karl of Hohenzollern-Sigmaringen as King Carol I, until 1947 with the abdication of King Michael I and the Romanian parliament's proclamation of the Romanian People's Republic.

Carol I or Charles I of Romania, was the monarch of Romania from 1866 to his death in 1914, ruling as Prince (Domnitor) from 1866 to 1881, and as King from 1881 to 1914. He was elected Prince of the Romanian United Principalities on 20 April 1866 after the overthrow of Alexandru Ioan Cuza by a palace coup d'état. In May 1877, Romania was proclaimed an independent and sovereign nation. The defeat of the Ottoman Empire (1878) in the Russo-Turkish War secured Romanian independence, and he was proclaimed King on 26 March [O.S. 14 March] 1881. He was the first ruler of the Hohenzollern-Sigmaringen dynasty, which ruled the country until the proclamation of a socialist republic in 1947.