Odesa, also spelled Odessa, is the third most populous city and municipality in Ukraine and a major seaport and transport hub located in the south-west of the country, on the northwestern shore of the Black Sea. The city is also the administrative centre of the Odesa Raion and Odesa Oblast, as well as a multiethnic cultural centre. As of January 2021, Odesa's population was approximately 1,010,537. On 25 January 2023, its historic city centre was declared a World Heritage Site and added to the List of World Heritage in Danger by the UNESCO World Heritage Committee in recognition of its multiculturality and 19th-century urban planning. The declaration was made in response to the bombing of Odesa during the Russian invasion of Ukraine, which has damaged or destroyed buildings across the city.

In classical antiquity a large Greek settlement existed at its location no later than the middle of the 6th century BC. It has been researched as a possible site of the ancient Greek settlement of Histria. The first chronicle mention of the Slavic settlement-port of Kotsiubijiv, which was part of the Grand Duchy of Lithuania, dates back to 1415, when a ship was sent from here to Constantinople by sea. After the Grand Duchy lost control, the port and its surroundings became part of the domain of the Ottoman Empire in 1529, under the name Hacibey, and remained in it until the Ottomans' defeat in the Russo-Turkish War (1787–1792). In 1794, a decree of the Russian empress Catherine II was issued to establish a navy harbor and trading place in Khadjibey, which was named Odessa soon after. From 1819 to 1858, Odesa was a free port. During the Soviet period, it was an important trading port and a naval base. During the 19th century, Odesa was the fourth largest city of the Russian Empire, after Moscow, Saint Petersburg and Warsaw. Its historical architecture is more Mediterranean 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.

Odesa is a warm-water port. The city of Odesa hosts both the Port of Odesa and Port Pivdennyi, a significant oil terminal situated in the city's suburbs. Another notable port, Chornomorsk, is located in the same oblast, to the south-west of Odesa. Together they represent a major transport hub integrating with railways. Odesa's oil and chemical processing facilities are connected to the Russian and other European networks by strategic pipelines. In 2000, the Quarantine Pier at Odesa Commercial Sea Port was declared a free port and free economic zone for a period of 25 years.

Odesa is sometimes called the Pearl by the Sea, the Southern Capital, Odesa-mama and the Humour Capital, as well as Southern Palmyra.

In 1795 the city was named Odessa in accordance with the Greek Plan of Empress Catherine II. Catherine's Secretary of State Adrian Gribovsky claimed in his memoirs that the name was his suggestion. Some expressed doubts about this claim, while others noted the reputation of Gribovsky as an honest and modest man. Odesa is located between the ancient Greek cities of Tyras and Olbia and it was named using a Slavic feminine form for the ancient Greek city of Odessos (Ancient Greek: Ὀδησσός; in Roman times, Odessus). This refers to the second ancient Odessos, founded between the end of the 5th and beginning of the 4th centuries BC (the first one, identified with modern Varna in Bulgaria, is the older of the two, founded c. 610 BC). The exact location of this ancient Odessos is unknown, but modern efforts have attempted to localize it 40 km northeast of Odesa, near the village of Koshary, Odesa Oblast., near the Tylihul Estuary.

Odessa, the transliteration of the name from Russian, was the traditional English spelling of the city's name favoured before Ukraine's independence in 1991 (similar to the spelling of Kiev versus Kyiv).

Odesa became the internationally standardized Latin-alphabet transliteration of the Ukrainian name according to the Ukrainian National romanization system, which was adopted for official use by Ukraine's cabinet in 2010, approved by the UN Group of Experts on Geographical Names in 2012, and adopted by the BGN/PCGN in 2019. This spelling appears in Encyclopædia Britannica and in dictionaries as the spelling for the Ukrainian city. The spelling Odesa has also been adopted as the Library of Congress Name Authority Heading. As noted by the Christian Science Monitor, many in the English-language media outlets historically spelled the city Odessa, even after changing the spelling of Kiev to Kyiv, but since the beginning of Russia's 2022 invasion of Ukraine more outlets and style guides have been shifting away from Russian transliterations.

Grand Duchy of Lithuania 1415–84
Ottoman Empire 1484–1789
Russian Empire 1789–1917