István ІІІ (Stephen; 1147–04.03.1172) – King of Hungary and Croatia, representative of the Árpád dynasty (1162–1172), son of Hungarian King Géza II and Yefrosynia, daughter of the Grand Prince of Kyiv Mstyslav Volodymyrovych.
The eldest son of Hungarian King Géza II and Yefrosynia, daughter of the Grand Prince of Kyiv Mstyslav Volodymyrovych the Great. His godfather was the King Louis VII of France. After his father’s death he became the King of Hungary. But he held out on the throne for only two months, as the Byzantine Emperor Manuel I Komnenus interfered in the internal affairs of the country and put on the Hungarian throne László II (Vladislav), István’s uncle. István with his mother hid in Pozsony. After the death of László II (Vladislav) Istvan IV, the pro-Byzantine ruler, came to the throne. In 1163 István III, having enlisted the support of Frederick I Barbarossa, Holy Roman Emperor, managed to regain the Hungarian throne and repel the Byzantine attack on Hungary. A peace treaty was signed with Byzantium, according to which the Szerémség region (part of medieval Hungary) passed to Byzantium, and the King’s younger brother Béla left to Byzantium as a hostage. In 1164 Komnenus unleashed a new war with Hungary and reconquered Croatia and Dalmatia (in 1167 they finally fell under the authority of Byzantium). In 1168 István III sought assistance in Europe in his struggle against Byzantium. First of all, he tried to enlist the support of Prince Yaroslav Osmomysl of Halych, but afterwards got married to Agnes of Austria, a daughter of Austrian Duke Henry II.
In the spring of 1172 he died because of disease (according to other version he was poisoned).