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.

Wallachia or Walachia is a historical and geographical region of modern-day Romania. It is situated north of the Lower Danube and south of the Southern Carpathians. Wallachia was traditionally divided into two sections, Muntenia and Oltenia. Dobruja could sometimes be considered a third section due to its proximity and brief rule over it. Wallachia as a whole is sometimes referred to as Muntenia through identification with the larger of the two traditional sections.