Stephen III, better known as Stephen the Great (Romanian: Ștefan cel Mare; [ˈʃtefan tʃel ˈmare]; died 2 July 1504), was Voivode of Moldavia from 1457 until his death. He was one of the most prominent rulers of late medieval Eastern Europe, noted for his long reign, military leadership and astute diplomacy. His efforts to preserve Moldavian autonomy from more powerful neighbouring states such as the Ottoman Empire, Poland and Hungary, as well as his aptitude for nation-building and repute as a protector of the Christian faith, made him into a national hero in both Romania and Moldova. He is canonised by the Romanian Orthodox Church.

In 1450, Stephen with his father, Bogdan II, repulsed the Polish invasion of Moldavia. However, his father was assassinated in 1451, with Stephen taking refuge in Transylvania. Following a period of internal strife, Stephen took the Moldavian throne in 1457, after his victory at the Battle of Orbic, with his cousin Vlad the Impaler's support. Peter Aaron, the murderer of Stephen's father, was deposed and sought refuge in Poland. Stephen attacked Polish territory and prevented Casimir IV Jagiellon, King of Poland, from supporting Peter, but eventually acknowledged Casimir's suzerainty in 1459. Stephen decided to recapture Chilia (now Kiliia in Ukraine), an important port on the Danube, which brought him into conflict with Hungary and Wallachia. He besieged the town during the Ottoman invasion of Wallachia in 1462, but was seriously wounded during the siege. Two years later, he captured the town. He promised support to the leaders of the Three Nations of Transylvania against Matthias Corvinus, King of Hungary, in 1467. Corvinus invaded Moldavia, which led to Battle of Baia, but Stephen repulsed him. Peter Aaron attacked Moldavia with Hungarian support in December 1470, but he was defeated by Stephen and executed, alongside the Moldavian boyars who still endorsed him.

Stephen restored old fortresses and built new ones, which improved Moldavia's defence system as well as strengthened central administration. Ottoman expansion threatened Moldavian ports in the region of the Black Sea. In 1473, Stephen stopped paying tribute (haraç) to the Ottoman sultan and launched a series of campaigns against Wallachia in order to replace its rulers, who had accepted Ottoman suzerainty, with his protégés. Stephen inflicted crushing defeats on Radu III at the Battles of Sochi and Vodna River, then occupied Bucharest. However, each prince who seized the throne with Stephen's support was soon forced to pay homage to the sultan. Stephen eventually defeated a large Ottoman army at the Battle of Vaslui during their invasion in 1475. He was referred to as Athleta Christi ("Champion of Christ") by Pope Sixtus IV, even though Moldavia's hopes for military support went unfulfilled. The following year, Ottoman Sultan Mehmed II routed Stephen in the Battle of Valea Albă, but the lack of provisions, failed Siege of Neamț Citadel and the outbreak of a plague forced him to withdraw from Moldavia. Taking advantage of a truce with Matthias Corvinus, the Ottomans captured Chilia and their Crimean Tatar allies Cetatea Albă (now Bilhorod-Dnistrovskyi in Ukraine) in 1484. Although Corvinus granted two Transylvanian estates to Stephen, the Moldavian prince paid homage to Casimir, who promised to support him to regain Chilia and Cetatea Albă. Stephen's efforts to capture the two ports ended in failure. From 1486, he again paid a yearly tribute to the Ottomans. During the following years, dozens of stone churches and monasteries were built in Moldavia, which contributed to the development of a specific Moldavian-style architecture.

In 1490, Stephen occupied Pokuttia. Casimir IV's successor, John I Albert, wanted to grant Moldavia to his younger brother, Sigismund, but Stephen's diplomacy prevented him from invading Moldavia for years. John Albert attacked Moldavia in 1497, but Stephen and his Hungarian and Ottoman allies routed the Polish army in the Battle of the Cosmin Forest. In 1499, Stephen routed the Ottoman army at Prut river, as they were returning from their raid on Poland. In 1500, Moldavian forces set Chilia and Cetatea Albă on fire. In 1502, Stephen and his Crimean Tatar allies destroyed the remnants of the Golden Horde at the Battle of Samara River. Stephen only acknowledged the loss of his ports to the Ottomans in 1503. During his last years, his son and co-ruler Bogdan III played an active role in government. Stephen's long rule represented a period of stability in the history of Moldavia. From the 16th century onwards, both his subjects and foreigners remembered him as a great ruler. Modern Romanians regard him as one of their greatest national heroes, and he also endures as a cult figure in Moldovenism. After the Romanian Orthodox Church canonised him in 1992, he is venerated as "Stephen the Great and Holy" (Ștefan cel Mare și Sfânt).

Stephen was the son of Bogdan, who was a son of Alexander the Good, Prince of Moldavia. Stephen's mother, Maria Oltea, was most probably related to the princes of Wallachia, according to historian Radu Florescu. The date of Stephen's birth is unknown, though historians estimate that he was born between 1433 and 1440. One church diptych records that he had five siblings: brothers Ioachim, Ioan, Christea; and sisters Sorea and Maria. Some of Stephen's biographers hypothesize that Cârstea Arbore, father of the statesman Luca Arbore, was the prince's fourth brother, or that Cârstea was the same as Ioachim. These links with the high-ranking Moldavian boyars are known to have been preserved through matrimonial connections: Maria, who died in 1485, was the wife of Șendrea, gatekeeper of Suceava; Stephen's other brother-in-law, Isaia, also held high office at his court.

The death of Alexander the Good in 1432 gave rise to a succession crisis that lasted more than two decades. Stephen's father seized the throne in 1449 after defeating one of his relatives with the support of John Hunyadi, Regent-Governor of Hungary. Stephen was styled voivode in his father's charters, showing that he had been made his father's heir and co-ruler. Bogdan acknowledged the suzerainty of Hunyadi in 1450. Stephen fled to Hungary after Peter III Aaron (who was also Alexander the Good's son) murdered Bogdan in October 1451.

Vlad Țepeș (who had lived in Moldavia during Bogdan II's reign) invaded Wallachia and seized the throne with the support of Hunyadi in 1456. Stephen either accompanied Vlad to Wallachia during the military campaign or joined him after Vlad became the ruler of Wallachia. According to reports from the 1480s, Stephen spent part of that interval in Brăila, where he fathered an illegitimate son, Mircea. With the assistance of Vlad, Stephen stormed into Moldavia at the head of an army 6,000 strong in the spring of 1457. According to Moldavian chronicles, "men from the Lower Country" (the southern region of Moldavia) joined him. The 17th-century Grigore Ureche wrote: "Stephen routed Peter Aaron at Doljești on 12 April, but Peter Aaron left Moldavia for Poland only after Stephen inflicted a second defeat on him at Orbic."

One widely accepted theory, based on Ureche, states that an assembly of boyars and Orthodox clergymen acclaimed Stephen the ruler of Moldavia at Direptate, a meadow near Suceava. According to scholar Constantin Rezachievici, this elective custom has no precedent before the 17th century, and appears superfluous in Stephen's case; he argues that it was a legend fabricated by Ureche. While this election remains uncertain, various historians agree that Teoctist I, Metropolitan of Moldavia, anointed Stephen prince. To emphasize the sacred nature of his rule, Stephen styled himself "By the Grace of God, ... Stephen voivode, lord (or hospodar) of the Moldavian lands" on 13 September 1457. His use of Christian devices for legitimization overlapped with a troubled context for Moldavian Orthodoxy: the attempted Catholic–Orthodox union had divided the Byzantine Rite churches into supporters and dissidents; likewise, the Fall of Constantinople had encouraged local bishops to consider themselves independent of the Patriarchy. There is a long-standing dispute about whether Teoctist was a dissenter, belonging to one of the several emancipated Orthodox jurisdictions, or a loyalist of Patriarch Isidore. Historian Dan Ioan Mureșan argues that the evidence is for the latter option, because Moldavia appears on the list of Patriarchate jurisdictions, and because Stephen, though he tested the Patriarch by sometimes using imperial titles such as tsar by 1473, was never threatened with excommunication.