John Sigismund Zápolya or Szapolyai (Hungarian: Szapolyai János Zsigmond; 7 July 1540 – 14 March 1571) was King of Hungary as John II from 1540 to 1551 and from 1556 to 1570, and the first Prince of Transylvania, from 1570 to his death. He was the only son of John I, King of Hungary, and Isabella of Poland. John I ruled parts of the Kingdom of Hungary with the support of the Ottoman Sultan Suleiman; the remaining areas were ruled by Ferdinand I of Habsburg, who also ruled Austria and Bohemia. The two kings concluded a peace treaty in 1538 acknowledging Ferdinand's right to reunite Hungary after John I's death, though shortly after John Sigismund's birth, and on his deathbed, John I bequeathed his realm to his son. The late king's staunchest supporters elected the infant John Sigismund king, but he was not crowned with the Holy Crown of Hungary.
Suleiman invaded Hungary under the pretext of protecting John Sigismund from Ferdinand. Buda, the capital of Hungary, fell to the Ottomans without opposition in 1541, but Suleiman allowed the dowager queen, Isabella, to retain the territory east of the river Tisza on John Sigismund's behalf. Isabella and John Sigismund moved to Lippa (now Lipova in Romania). Before long, they took up residence in Gyulafehérvár in Transylvania (Alba Iulia in Romania). John Sigismund's realm was administered by his father's treasurer, George Martinuzzi, who sought to reunite Hungary under the rule of Ferdinand. Martinuzzi forced Isabella to renounce her son's realm in exchange for two Silesian duchies and 140,000 florins in 1551. John Sigismund and his mother settled in Poland, but she continued to negotiate for John Sigismund's restoration with Ferdinand's enemies.
Ferdinand was unable to protect eastern Hungary against the Ottomans. At Suleiman's urging, the Transylvanian Diet in 1556 persuaded John Sigismund and his mother to return to Transylvania, where she ruled her son's realm until her death in 1559. A wealthy lord, Melchior Balassa, rebelled against John Sigismund in late 1561, and Ferdinand gained control of most counties outside Transylvania. The Székely people, whose liberties had been restricted in the 1550s, also rose up against John Sigismund, but he crushed the rebellion. During the ensuing war against the Habsburgs, the Ottomans supported John Sigismund, and he paid homage to Suleiman in Zemun in 1566. The 1568 Treaty of Adrianople concluded the war, confirming John Sigismund in the eastern territories of the medieval Kingdom of Hungary (Transylvania and "Partium").
John Sigismund initiated a series of theological debates among the representatives of the concurring theological schools of the Reformation in the 1560s. He converted from Catholicism to Lutheranism in 1562 and from Lutheranism to Calvinism in 1564. Around five years later, accepting the Anti-Trinitarian views of his physician, Giorgio Biandrata and court preacher Ferenc Dávid, he became the only Unitarian monarch in history. In 1568, the Diet passed the Edict of Torda (now Turda in Romania), which emphasized that "faith is a gift of God" and prohibited the persecution of people for religious reasons. The edict expanded the limits of freedom of religion beyond the standards of late 16th-century Europe. John Sigismund abandoned the title "elected king of Hungary" in the Treaty of Speyer in 1570. Thereafter, he styled himself "Prince of Transylvania and Lord of Parts of the Kingdom of Hungary". He died childless. The Catholic Stephen Báthory succeeded him.
John Sigismund's father, John Zápolya, was the wealthiest Hungarian lord in the early 16th century. After the Ottoman Sultan, Suleiman the Magnificent, inflicted a crushing defeat on the Hungarian army in the Battle of Mohács, the majority of the noblemen elected John Zápolya king in 1526. However, a group of influential lords proclaimed Ferdinand I, Archduke of Austria, king in the same year. Hungary fell into a civil war that lasted for decades.
John paid homage to Suleiman at Mohács in 1529 to secure Ottoman support against Ferdinand. However, neither John nor Ferdinand could win control of the whole country during the next years. To conclude the civil war, the two kings' envoys signed the Treaty of Várad on 24 February 1538, which confirmed both kings' right to retain the lands that they held. John, who was childless, also acknowledged Ferdinand's right to take control of his realm (the central and eastern part of the Kingdom of Hungary) after his death. John also stipulated, if he fathered a son, his son would inherit his ancestral domains. Ferdinand, however, proved unable to protect John's realm against an Ottoman invasion. At age 52 John married Isabella Jagiellon, the 20-year-old daughter of Sigismund I the Old, King of Poland, on 2 March 1539. The humanist scholars Paolo Giovio and Antun Vrančić emphasized that Isabella was one of the most educated women of their age.
John Sigismund was born in Buda on 7 July 1540. On learning of his birth, his father, who was on campaign in Transylvania, rode to his soldiers' camp to inform them of the good news. The following day John fell ill, and he died on 21 or 22 July. Before his death he persuaded those present at his death bed to take an oath that they would prevent the transfer of his realm to Ferdinand.
Soon after John Zápolya died, his treasurer, George Martinuzzi, hurried to Buda to secure John Sigismund's inheritance. On Martinuzzi's proposal, the Diet of Hungary elected John Sigismund king on 13 September 1540, but he was not crowned with the Holy Crown of Hungary. The Diet proclaimed Queen Isabella and George Martinuzzi, along with two powerful lords, Péter Petrovics and Bálint Török, the guardians of the infant monarch.
In August, Ferdinand's envoys had demanded the transfer of the late John Zápolya's realm to Ferdinand in accordance with the Treaty of Várad. Peter Perényi, who had been the commander of Zápolya's troops in Upper Hungary, and Franjo Frankopan, Archbishop of Kalocsa, soon deserted to Ferdinand. The wealthy Stephen Majláth expelled most of John Sigismund's supporters from Transylvania in an attempt to seize the province for himself. Ferdinand's envoy, Hieronymus Łaski, informed Suleiman of the Treaty of Várad, asking the sultan to consent to the unification of Hungary under Ferdinand's rule. Instead, the sultan stated that he supported John Sigismund and had Łaski arrested.
Ferdinand's army seized Visegrád, Vác, Pest, Tata and Székesfehérvár in October, but could not capture Buda. His military commander, Wilhelm von Roggendorf, again laid siege to Buda on 4 May 1541. Suleiman left Istanbul at the head of a large army in June to take advantage of the new civil war in Hungary. On his command, Petru Rareș, Prince of Moldavia, captured Stephen Majláth and forced the Transylvanian Diet to swear fealty to John Sigismund in late July. Roggendorf lifted the siege of Buda before Suleiman reached the town on 26 August.
Suleiman said that he had come to protect John Sigismund's interests, but also announced that he wanted to see the infant king, because he had heard rumours about Isabella's having actually given birth to a daughter. Six Hungarian lords (including George Martinuzzi and Bálint Török) accompanied John Sigismund to the sultan's camp on 29 August. During the meeting, janissaries entered Buda, saying that they wanted to see the town. This turned out to be a trick that enabled them to seize the capital of Hungary without opposition. Bálint Török was captured in the sultan's camp. Suleiman declared that John Sigismund could retain the territories to the east of the river Tisza in exchange for a yearly tribute of 10,000 florins.
Isabella and Martinuzzi left Buda on 5 September 1541, taking John Sigismund and the Holy Crown with them. She and her son settled in Lippa, which was the center of an old domain of the Zápolya family. The delegates of the counties of John Sigismund's realm met in Debrecen on 18 October. They swore fealty to him and acknowledged the sultan's suzerainty. Martinuzzi signed a treaty with Ferdinand I's representative, Caspar Serédy, in Gyalu (now Gilău in Romania) on 29 December. According to the Treaty of Gyalu, Hungary was to be reunited under the rule of Ferdinand, but John Sigismund's right to the Zápolyas' domains in Upper Hungary was confirmed.