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.

The United Principalities of Moldavia and Wallachia, commonly called United Principalities or Wallachia and Moldavia, was the personal union of the Principality of Moldavia and the Principality of Wallachia. The union was formed 5 February [O.S. 24 January] 1859 when Alexandru Ioan Cuza was elected as the Domnitor of both principalities. Their separate autonomous vassalage in the Ottoman Empire continued with the unification of both principalities. On 3 February [O.S. 22 January] 1862, Moldavia and Wallachia formally united to create the Romanian United Principalities, the core of the Romanian nation state.