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.

Principe Domnitor was the official title of the ruler of Romania between 1862 and 1881. It was usually translated as "prince regnant" in other languages and less often as "grand duke". "Domnitor" is an adjective derived from the Romanian word "domn" and, in turn, from the Latin "Dominus". The title Domn had been in use since the Middle Ages and it is also the Romanian equivalent to the Slavic Hospodar. Moldavian and Wallachian rulers had used this term for their title of authority as the head of state, while "voievod" represented the military rank as the head of the army.