Chersonesus, contracted in medieval Greek to Cherson (Χερσών), was an ancient Greek colony founded approximately 2,500 years ago in the southwestern part of the Crimean Peninsula. Settlers from Heraclea Pontica in Bithynia established the colony in the 6th century BC.

The ancient city was located on the shore of the Black Sea on the outskirts of present-day Sevastopol on the Crimean Peninsula, where it is referred to as Khersones. The site is part of the National Preserve of Tauric Chersonesos. The name Chersonesos in Greek means "peninsula" and aptly describes the site on which the colony was established. It should not be confused with the Tauric Chersonese, a name often applied to the whole of the southern Crimea.

During much of the classical period, Chersonesus operated as a democracy ruled by a group of elected archons and a council called the Damiorgi. As time passed, the government grew more oligarchic, with power concentrated in the hands of the archons. A form of oath sworn by all the citizens from the 3rd century BC onwards has survived to the present day. In 2013 UNESCO listed Chersonesus as a World Heritage Site.

In the 5th century BC, Dorians from Heraclea Pontica on the Black Sea coast of Asia Minor founded the sea port of Chersonesos in southwestern Crimea (outside modern Sevastopol). It was a site with good deep-water harbors located at the edge of the territory of the indigenous Taurians. During much of the Classical Period, Chersonesus was a democracy ruled by a group of elected archons and a council called the Demiurgi. As time passed the government grew more oligarchic, with power concentrated in the hands of the archons. Up to the middle of the 4th century BC, Chersonesos remained a small city. It then expanded to lands in northwest Crimea, incorporating the colony of Kerkinitida and constructing numerous fortifications.

After defending itself against the Bosporan Kingdom, and the native Scythians and Tauri, and even extending its power over the west coast of the peninsula, it was compelled to call in the aid of Mithradates VI and his general Diophantus, c. 110 BC, and submitted to the Bosporan Kingdom. It was subject to Rome and received a garrison from the middle of the 1st century BC until the 370s AD, when it was captured by the Huns.

It became a Byzantine possession during the Early Middle Ages and withstood a siege by the Göktürks in 581. Byzantine rule was slight: there was a small imperial garrison more for the town's protection than for its control and it exercised a measure of self-government. It was useful to Byzantium in two ways: it was an observation point to watch the barbarian tribes, and its isolation made it a popular place of exile for those who angered the Roman and later Byzantine governments. Among its more famous "inmates" were Pope Clement I and Pope Martin I, and the deposed Byzantine Emperor Justinian II.

According to Theophanes the Confessor and others, Chersonesus was the residence of a Khazar governor (tudun) in the late 7th century. Between approximately 705 and 840, the city's affairs were managed by elected officials called babaghuq, meaning "father of the city".

In 833, Emperor Theophilus sent the nobleman Petronas Kamateros, who had recently overseen the construction of the Khazar fortress of Sarkel, to take direct control over the city and its environs, establishing the theme of Klimata/Cherson. It remained in Byzantine hands until the 980s, when it reportedly fell to Vladimir the Great of the Kievan Rus'. Vladimir agreed to evacuate the fortress only if Basil II's sister Anna Porphyrogeneta would be given him in marriage. The demand caused a scandal in Constantinople. As a pre-condition for the marriage settlement, Vladimir was baptized here in 988, thus paving the way to the Baptism of Kievan Rus'. Thereafter Korsun' was evacuated.