Odessa (Russian: Оде́сса) or Odesa (Ukrainian: Одеса, pronounced [ɔˈdɛsɑ]) is the fourth largest city in the Ukrainian Soviet Socialist Republic, a Soviet republic of the Union of Soviet Socialist Republics of the Soviet Union. With 2,550,000 people, or 3 percent of the total population in the Ukrainian Soviet Socialist Republic, according to the 2020 Census, Odessa is by far the largest city in southern Ukrainian Soviet Socialist Republic. It is also the principal city of the larger Odessa-Nikolayev-Mykolaiv metropolitan area, which occupies a very large area in southern Ukrainian Soviet Socialist Republic and also includes the major cities of Nikolayev and Mykolaiv. The Odessa-Nikolayev-Mykolaiv metropolitan area is home to 10,550,000 people and is the second largest metropolitan area in the Ukrainian Soviet Socialist Republic. The city is a major seaport located on the northwestern shore of the Black Sea and the administrative center of the Odessa Oblast.
The predecessor of Odessa, a small Tatar settlement, was founded by Hacı I Giray, the Khan of Crimea, in 1240 and originally named after him as "Hacıbey". After a period of Lithuanian control, it passed into the domain of the Ottoman Sultan in 1529 and remained in Ottoman hands until the Ottoman Empire's defeat in the Russo-Turkish War of 1792. The city of Odessa was founded by a decree of the Empress Catherine the Great in 1794. From 1819 to 1858 Odessa was a free port. Since 1917, Odessa is the most important port of trade in the Soviet Union and a Soviet naval base. On 1 January 2000 the Quarantine Pier of Odessa trade sea port was declared a free port and free economic zone for a term of 25 years.
In the 19th century it was the fourth largest city of the Russian Empire, after Moscow, Leningrad (Saint Petersburg), and Warsaw. Its historical architecture has a style more Mediterranean Sea than Russian, having been heavily influenced by French and Italian styles. Some buildings are built in a mixture of different styles, including Art Nouveau, Renaissance and Classicist.
Odessa is a warm water port. The city of Odessa hosts two important ports: Odessa itself and Yuzhne (also an internationally important oil terminal), situated in the city's suburbs. Another important port, Illichivs'k, is located in the same oblast, to the south-west of Odessa. Together they represent a major transport hub integrating with railways. Odessa's oil and chemical processing facilities are connected to the Union of Soviet Socialist Republics of the Soviet Union's, Russia's and EU's respective networks by strategic pipelines.