It happened when Prince Vladimir of Kiev converted to Christianity, the Foreign Ministry proclaimed in a tweet
Ukraine has "always" been a friend of Europe and effectively decided to become part of the European Union at the end of the 10th century, the country's Foreign Ministry has said.
"Ukraine and Europe were always friends. In 988, Prince Volodymyr converted the Kyiv Rus to Christianity and decided [that] we need to join the EU," the ministry tweeted on Friday, referring to the prince and the country's capital by their Ukrainian names.
The ministry added that this was the moment when Ukraine "became part of the Western world."
The prince, also known as Vladimir the Great and Vladimir the Saint, ruled from Kiev between 978 and 1015. His realm was called Kievan Rus, the first historical state from which Russia, Ukraine and Belarus trace their origin.
Vladimir had been an adherent of paganism earlier in his reign, but, inspired by the Byzantine Empire, eventually chose to convert, which began the Christianization of the domain that he ruled.
The EU, meanwhile, traces its history from the European Coal and Steel Community, founded in 1951, and the European Economic Community, founded in 1957.
Ukraine formally applied to join the EU in February, shortly after Russia launched its military operation in the neighboring country.