Southeast Europe or Southeastern Europe (SEE) is a geographical sub-region of Europe, consisting primarily of the region of the Balkans, as well as adjacent regions and archipelagos. There are overlapping and conflicting definitions of the region, due to political, economic, historical, cultural, and geographical considerations.

Sovereign states and territories that may be included in the region are Albania, Bosnia and Herzegovina, Bulgaria, Croatia (alternatively placed in Central Europe), Greece (alternatively placed in the broader region of Southern Europe), Kosovo, Montenegro, North Macedonia, Romania (alternatively placed in Eastern Europe), Serbia, and the European part of Turkey (alternatively placed in the broader region of Southern Europe, also in Western Asia with the rest of the country). Sometimes, Cyprus (most often placed in West Asia), Hungary (most often placed in Central Europe), Moldova (most often placed in Eastern Europe) and Slovenia (most often placed in Central Europe) are also included due to cultural or historical factors and affiliation.

The largest cities of the region are Istanbul, Athens, Bucharest, Sofia, and Belgrade.