Union of Brest (Belarusian: Берасцейская унiя, Ukrainian: Берестейська унія, Polish: Unia brzeska) or Union of Brześć refers to the 1595-1596 decision of the Ruthenian Church of Rus', the "Metropolia of Kiev-Halych and all Rus'", to break relations with the Patriarch of Constantinople and enter into communion with and place itself under the authority of the Pope of Rome.

At the time, this church included most Ukrainians and Belarusians who lived in the Polish-Lithuanian Commonwealth. The hierarchs of the Kievan church gathered in synod in the city of Brest composed 33 articles of Union, which were accepted by the Pope of Rome. At first widely successful, within several decades it had lost much of its initial support, mainly due to its enforcement on the Orthodox parishes, which stirred several massive uprisings, particularly the Khmelnytskyi Uprising, of the Zaporozhian Cossacks and the Commonwealth lost Ukraine in result. By the end of the 18th century, through parallel persecutions of Orthodoxy, it would become the sole church for Ruthenians living in the Commonwealth. After the Partitions of Poland, which saw all but Galicia enter into the Russian Empire, within decades all but the Chełm Eparchy, would revert to Orthodoxy. The latter would be forcibly converted in 1875. In Austrian Galicia, however, the church underwent a transformation, from being a tool of oppression of the Polish efforts, to one of the founding cornerstones of the Ukrainian national awakening in the 19th century, first as a centre for Western Ukrainian Russophilia and then, for Ukrainophilia. It would remain a vital centre for Ukrainian culture during the Second Polish Republic and Ukrainian nationalism during the Second World War. Although between 1946 and 1989 it was forcibly adjoined to the Russian Orthodox Church by Soviet authorities, it would come out of the catacombs as the Ukrainian Greek Catholic Church in the prelude to Ukraine's independence.