The Baptism of Poland was the event in 966 that signified the beginning of the Christianization of Poland, commencing with the baptism of Mieszko I, who was the first ruler of the Polish state. The next significant step in Poland's adoption of Christianity was the establishment of various ecclesiastical organs in the country during the 10th and 11th centuries. This included the building of cathedrals and the appointment of clergy. This date should not be confused as the independence day of Poland.

In adopting Christianity as the state religion, Mieszko sought to achieve several personal goals. He saw Poland's baptism as a way of strengthening his hold on power, as well as using it as a unifying force for the Polish people. The exact place of Mieszko's baptism is disputed, historians having alternately argued that Gniezno, Poznań, Cologne or even Rome was the site of the event. Mieszko's action proved highly influential; by the 13th century, Roman Catholicism had become the dominant religion in Poland, although pagan resistance denoted with the historic term 'Pagan reaction' remained until the 1040's.