Southern Dobruja or South Dobruja (Bulgarian: Южна Добруджа, Yuzhna Dobrudzha or simply Добруджа, Dobrudzha) is an area of northeastern 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. When it was a part of Romania from 1913 to 1940 it was known in Romanian as Dobrogea de sud, the Cadrilater ("Quadrilateral"), or Dobrogea Nouă ("New Dobruja").
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 1878 and part of the independent Bulgarian state from 1908 until Bulgaria's defeat in the Second Balkan War, when the region was ceded to Romania under the Treaty of Bucharest (1913).
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, as well as Romanians from Wallachia, which led to claims that the reforms had a nationalist purpose.
On 7 September 1940 Southern Dobruja was restored to Bulgaria under the Treaty of Craiova. The treaty was followed by a mandatory population exchange: about 110,000 Romanians (almost 95% of whom settled there after 1913) were forced to leave Southern Dobruja, whereas 77,000 Bulgarians had to leave Northern Dobruja. Only a few hundred Romanians and Aromanians are now left in the region.