Alexandru Ioan Cuza (pronounced [aleksandru ioa kuza] (listen), or Alexandru Ioan I, also anglicised as Alexander John Cuza; 20 March 1820 15 May 1873) was the first domnitor (Ruler) of the Romanian Principalities through his double election as prince of Moldavia on 5 January 1859 and prince of Wallachia on 24 January 1859, which resulted in the unification of both states. He was a prominent figure of the Revolution of 1848 in Moldavia. Following his double election, he initiated a series of reforms that contributed to the modernization of Romanian society and of state structures.

As ruler of the Romanian Principalities, he supported a political and diplomatic activity for the recognition of the union of Moldavia and Wallachia by the suzerain Ottoman Empire and achieved constitutional and administrative unity between Moldavia and Wallachia in 1862, when the Romanian Principalities officially adopted the name Romanian United Principalities with a single capital at Bucharest, a single national assembly and a single government.

Alexandru's reform policies alienated a large coalition of conservatives and radical liberals, for the most part landowners and business owners. On 22 February 1866, he was forced to abdicate and leave the country. Today, he is often considered one of the founders of the modern Romanian state and a national hero of Romania.

The United Principalities of Moldavia and Wallachia (Romanian: Principatele Unite ale Moldovei și Țării Românești), commonly called United Principalities, was the personal union of the Principality of Moldavia and the Principality of Wallachia, formed on 5 February [O.S. 24 January] 1859 when Alexandru Ioan Cuza was elected as the Domnitor (Ruling Prince) of both principalities, which were autonomous but still vassals of the Ottoman Empire and which resulted in 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.In February 1866, Prince Cuza was forced to abdicate and go into exile by a political coalition led by the Liberals; the German Prince Karl of Hohenzollern-Sigmaringen was offered the Throne and, on 22 May [O.S. 10 May] 1866 he entered Bucharest for the first time. In July the same year, a new constitution came into effect, giving the country the name of Romania; internationally, this name was used only after 1877, since at the time the foreign policy of the state was drafted by the Ottomans. Nominally, the new state remained a vassal of the Ottoman Empire. However, by this time the suzerainty of the Sublime Porte had become a legal fiction. Romania had its own flag and anthem, and conducted its own foreign policy. From 1867, it had its own currency as well.

On 21 May [O.S. 9 May] 1877, Romania proclaimed itself fully independent; the proclamation was sanctioned by the Domnitor the following day. Four years later, the 1866 constitution was modified so the Romania became a kingdom, on 22 May [O.S. 10 May] 1881, Domnitor Carol I was crowned as the first King of Romania. After the First World War, Transylvania and other territories were also included.

For its triple symbolic meaning, the date of May 10 was celebrated as Romania's National Day until 1948, when Romanian and Soviet Communists installed the republic via coup d'état on December 30, 1947.