The Reformation in Sweden is generally regarded as having begun in 1527 during the reign of King Gustav I of Sweden, but the process was slow and was not definitively decided until the Uppsala Synod of 1593, in the wake of an attempted Counter-Reformation during the reign of John III (1568–1592).
The Swedish Reformation caused the Church of Sweden to break with the Roman Catholic Church. It is considered to mark the end of the Swedish Middle Ages. The Reformation resulted in both Sweden and Finland becoming Protestant countries, as the latter formed an integral part of Sweden at the time.