The Polish Reformed Church, officially called the Evangelical Reformed Church in the Republic of Poland (Polish: Kościół Ewangelicko-Reformowany w RP) is a historic Calvinistic Protestant church in Poland established in the 16th century, still in existence today.

According to Poland's Central Statistical Office, the Polish Reformed Church has 3,461 members (2015). The majority of church members live in central Poland; in 2014 out of a total number of 3464 adherents, 1800 lived in Łódź Voivodeship and 1000 in the city of Warsaw. There are eight congregations in Poland:

Furthermore, emerging congregations exist in some other cities, including Poznań, Wrocław, and Gdańsk. In 2003, the Church ordained its first female minister and two more female students are in training. The Polish Reformed Church is a minority church in Poland, where roughly 71% of the people were Catholics in 2021.

The Polish Reformed movement goes back to the half of the 16th century when the teachings of Swiss Reformers like Zwingli and Calvin began to make their way to Poland. Earlier, Lutheranism had made way to Poland, especially in the cities. A great boost to the Calvinist Reformation movement happened when in 1525, the devout Catholic king Sigismund I the Old (1506–48) accepted as his vassal in Ducal Prussia, the Lutheran prince Albert I, Duke of Prussia, thus creating the first Protestant country in the World. Though the king opposed "new thought", humanists all across the Polish-Lithuanian union began studying Calvinist theology. The most celebrated and influential group was found in the country's capital Kraków, where they flocked around the book printer and vendor Jan Trzecielski grouping nobles, burghers, professors, priests. The first Calvinist church service was held in 1550 in Pińczów a little town nearby Kraków, where the local noble owner converted to the Reformed Faith, expelled the monks, ’purging’ the city church. Other nobles soon followed suit and the first Calvinist synod in Lesser Poland was held in 1554 in Słomniki, close to Kraków. Thus, the Lesser Poland Brethren (Jednota Małopolska) was formed. ^{[citation needed]}

In the meantime, in the North of Poland, another Calvinist church was formed. The Czech Brethren, persecuted by the Czech king Ferdinand I Habsburg fled to Greater Poland (1548), where they settled in the estates of the local aristocrats whom they very quickly converted to their faith. The number of their congregations quickly swelled from 20 in 1555 to 64 in 1570. Their main centre was the city of Leszno, where they were settled under the patronage of the devotedly Reformed Leszczyński family. Thus the Czech Brethren, also called the Greater Poland Brethren (Jednota Wielkopolska), was formed. The Greater Poland and the Lesser Poland Brethren did try to cooperate more closely and even signed in 1555 a Union agreement but the Lesser Poland's Reformed nobles who formed the bulwark of the church members found the Czechs to be too hierarchical and undemocratic, and in the end the Lesser Poland Brethren became a strongly synodal structure, while the Greater Poland church became more Presbyterian. ^{[citation needed]}

The Reformation in the Grand Duchy of Lithuania (today's Lithuania, Belarus, and Ukraine) date to 1552 when the local aristocrat Mikołaj "the Black" Radziwiłł received a Reformed preacher, although some of Reformation ideas were known in Sigismund II Augustus palace because of returned educated Lithuanian Abraomas Kulvietis, who had founded school and taught children in Lutheran manner. He was generally unpopular among the Catholic hierarchy because of his Lutheran beliefs, and when the queen was away in 1542 Abraomas was forced to leave the country. Soon he (Radziwiłł "The Black") was followed by his cousin Mikołaj "the Red" Radziwiłł and other aristocrats. The reformer Jan Łaski worked for King Sigismund II from 1556 onwards. The first synod was held in 1557, and two years later the Lithuanians signed a Union agreement with the Lesser Poland Brethren. A huge number of converts were attracted from Orthodox nobility. While the nobles used Polish in church services, an effort was made to convert the Lithuanian-speaking peasants and serfs, but since Lithuanian did not have a written form till the second half of the 19th century, Polish stayed as the official church language. Thus, the Lithuanian Brethren (Jednota Litewska) came into being. ^{[citation needed]}

In 1556, John a Lasco (Jan Łaski) returned from Western Europe to help with the organisation of the Polish Reformed church. Seeing that the new king Sigismund II Augustus was sympathetic to the Calvinist cause, he tried to write a confession that would be agreeable not only to all the three Calvinist churches but to the Lutherans as well. Unfortunately, exhausted from overwork, he died in 1560, having achieved only the consolidation of the Lesser Reformed Brethren, which shortly afterwards was weakened by the split of the Unitarians (1563). In the same year, the Second Helvetic Confession was translated to Polish and was adopted by the Lithuanian and Lesser Poland Brethren. ^{[citation needed]} Łaski has been called the ‘Father of the Polish Reformed Church’.

In a posthumous tribute to John a Lasco, the Czech Brethren, the two Calvinist and Lutheran churches in Poland agreed in 1570 to the Confession of Sandomir (Konfesja Sandomierska), which was an irenic translation of the Second Helvetic Confession and in theory formed one, united, Protestant church. The strength of the Polish Protestants was shown when in 1573 a law was passed foreboding any persecution based on religion, an act unprecedented in Europe of that time. The Protestants formed also over 65% members of the Lower and just about a half of the Upper Houses of Parliament. ^{[citation needed]}