Andrew III the Venetian (Hungarian: III. Velencei András, Croatian: Andrija III. Mlečanin, Slovak: Ondrej III., Italian: Andrea III il veneziano; c. 1265 – 14 January 1301) was King of Hungary and Croatia between 1290 and 1301. His father, Stephen the Posthumous, was the posthumous son of Andrew II of Hungary although Stephen's older half brothers considered him a bastard. Andrew grew up in Venice, and first arrived in Hungary upon the invitation of a rebellious baron, Ivan Kőszegi, in 1278. Kőszegi tried to play Andrew off against Ladislaus IV of Hungary, but the conspiracy collapsed and Andrew returned to Venice.

Being the last male member of the House of Árpád, Andrew was elected king after the death of King Ladislaus IV in 1290. He was the first Hungarian monarch to issue a coronation diploma confirming the privileges of the noblemen and the clergy. At least three pretenders—Albert of Austria, Mary of Hungary, and an adventurer—challenged his claim to the throne. Andrew expelled the adventurer from Hungary and forced Albert of Austria to conclude a peace within a year, but Mary of Hungary and her descendants did not renounce their claim. The Hungarian bishops and Andrew's maternal family from Venice were his principal supporters, but the leading Croatian and Slavonian lords were opposed to his rule.

Hungary was in a state of constant anarchy during Andrew's reign. The Kőszegis, the Csáks, and other powerful families autonomously governed their domains, rising up nearly every year in open rebellion against Andrew. With Andrew's death, the House of Árpád became extinct. A civil war ensued which lasted for more than two decades and ended with the victory of Mary of Hungary's grandson, Charles Robert.

Andrew was the son of Stephen the Posthumous, the self-styled Duke of Slavonia, and his second wife, Tomasina Morosini. Andrew's father was born to Beatrice d'Este, the third wife of Andrew II of Hungary, after the king's death. However, Andrew II's two elder sons, Béla IV of Hungary and Coloman of Halych, accused Beatrice of adultery and refused to acknowledge Stephen as their legitimate brother. Andrew's mother, Tomasina, was the daughter of wealthy Venetian patrician Michele Morosini.

The exact date of Andrew's birth is unknown. According to historians Tibor Almási, Gyula Kristó, and Attila Zsoldos, he was born in about 1265. Stephen the Posthumous nominated two of his wife's kinsmen, including her brother Albertino Morosini, as Andrew's guardians before his death in 1272.

Andrew came to Hungary for the first time in 1278 at the invitation of a powerful lord, Ivan Kőszegi. Kőszegi wanted to play Andrew off against Ladislaus IV of Hungary. Andrew, who was the only male member of the royal family besides the king, adopted the title of "Duke of Slavonia, Dalmatia and Croatia" and marched as far as Lake Balaton. Andrew achieved nothing, however, and went back to Venice in autumn.

Andrew returned to Hungary at the beginning of 1290. On this occasion, Lodomer, Archbishop of Esztergom, also urged him to come, since the archbishop wanted to dethrone the excommunicated Ladislaus IV with the assistance of Ivan Kőszegi. Before Andrew was successful, Arnold Hahót, an enemy of the Kőszegis, invited him to the fort of Štrigova and captured him. Hahót sent Andrew to Vienna, where Albert I, Duke of Austria, held him in captivity.

Three Cuman assassins murdered Ladislaus IV on 10 July 1290, and Archbishop Lodomer subsequently dispatched two monks to Vienna to inform Andrew of the king's death. With the monks' assistance, Andrew left his prison in disguise and hastened to Hungary.