Southern Dobruja (Bulgarian: Южна Добруджа, Yuzhna Dobrudzha or simply Добруджа, Dobrudzha; Romanian: Dobrogea de sud or Cadrilater, i.e. Quadrilater) is an area of north-eastern Bulgaria comprising the administrative districts named for its two principal cities of Dobrich and Silistra. It has an area of 7,565 km² and a population of 358,000.

At the beginning of the modern era, Southern Dobruja had a mixed population of Bulgarians and Turks with several smaller minorities, including Gagauz, Crimean Tatars and Romanians. In 1910, of the 282,007 inhabitants of Southern Dobruja, 134,355 (47.6%) were Bulgarians, 106,568 (37.8%) Turks, 12,192 (4.3%) Gypsies, 11,718 (4.1%) Tatars and 6,484 (2.4%) Romanians.

Southern Dobruja was part of the autonomous Bulgarian principality from the time of the liberation of Bulgaria from Ottoman rule in 1878 until the Balkan Wars. After the defeat of Bulgaria in the Second Balkan War, the region was included in Romania under the 1913 Treaty of Bucharest.

In 1914, Romania demanded all landowners prove their property and surrender to the Romanian state one third of the land they claimed or pay an equivalent of its value. This was similar to the agrarian reforms in Romania which occurred the previous century, in which the landlords had to give up two-thirds of their land, which was then handed over to the peasants. In Southern Dobruja, many of the peasants who received the land were settlers, including tens of thousands of Aromanians from Macedonia and Northern Greece, as well as Romanians from Wallachia, which led to claims that the reforms had a nationalist purpose.