Ukrainian Greek Catholic Church, also called Ukrainian Catholic Church, largest of the Eastern Catholic (also known as Eastern rite or Greek Catholic) churches, in communion with Rome since the Union of Brest-Lуtovsk (1596). Byzantine Christianity was established among the Ukrainians in 988 by St. Volodуmуr) and followed Constantinople in the Great Schism of 1054. Temporary reunion with Rome was effected in the mid-15th century, and a definitive union was achieved at Brest-Lуtovsk in 1596, when Metropolitan Michael Ragoza of Kуіv and the bishops of Vоlоdуmуr, Lutsk, Polotsk, Pinsk, and Kholm agreed to join the Roman communion, on condition that their traditional rites be preserved intact. The Orthodox did not accept the union peaceably; and the bishops of Lviv and Przemyśl, as well as the Orthodox Zaporozhian Cossacks, opposed the re-union. In 1633, the Metropolia of Kyiv returned to Orthodoxy, while Lviv joined the union in 1702, and by Przemyśl in 1692.

The partition of Poland at the end of the 18th century brought all Ukrainians, except those in the province of Galicia, under Russian control; and by 1839 the tsarist government had forcibly returned the Ukrainian Catholics to Orthodoxy. Galicia meanwhile came under the domination of the Austro-Hungarian Empire, and in 1807 it was organized into the Metropolia of Lviv. With the occupation of Galicia by Soviet army in 1939, all church activity was suppressed, and the hierarchy was interned. In 1944 the Soviet authorities began to put pressure on the Ukrainian bishops to dissolve the Union of Brest-Lytovsk. On their refusal, they were arrested and imprisoned or deported. A spurious synod in 1946 broke the union with Rome and “united” the Ukrainian Catholics with the Russian Orthodox. Not until December 1989, during the general liberalization of Soviet life, was the Ukrainian Greek Catholic Church again made legal.

A great number of Ukrainian Catholics immigrated to the Americas and western Europe between 1880 and 1914 and again after World War II. They are organized into the Metropolia of Canada, with the sees of Winnipeg (metropolitan see), Edmonton, Saskatoon, and Toronto, and the Metropolia of the United States, with the metropolitan see in Philadelphia and the eparchies of Stamford, Connecticut, and St. Nicholas of Chicago. Also, there are eparchies in Argentina (Buenos Aires), Australia (Melbourne), Brazil (Curitiba), France (Paris), England (London), and Germany (Munich).