Otto Orseolo (Italian: Ottone Orseolo, 993−1032) born Pietro Orseolo, was the Doge of Venice from 1009 to 1026 and again from 1030 to 1032. He was the third son of doge Pietro II Orseolo and Maria Candiano, whom he succeeded at the age of sixteen, becoming the youngest doge in Venetian history.

During his dogeship, Otto strengthened the influence of the House of Orseolo within Venice and continued his father's policy of expanding Venetian authority in Dalmatia. His rule was marked by tensions with the Holy Roman Empire over the dispute between the patriarchates of Grado and Aquileia, as well as by internal opposition to his family's dominance of Venetian politics. Deposed in 1026 by a revolt led by Domenico Flabanico, Otto was banished and went into exile in Constantinople. In 1030, his brother Orso, Patriarch of Grado, overthrew doge Pietro Barbolano in a coup and assumed control of Venice as regent, recalling Otto to reclaim the dogeship. Otto, however, died on his journey back to Venice in 1032.

Born in Venice in 993, he was the third son of doge Pietro II Orseolo and his wife Maria Candiano, daughter of doge Vitale Candiano. His original name was Pietro, but in 996, while Emperor Otto III was residing in Verona and granting privileges to Venice in the March of Verona, he requested that Pietro II Orseolo send his third son to Verona, where he acted as the boy's sponsor at his confirmation. In the Emperor's honour, Pietro took the name Otto. In 1004, Otto, in the company of his eldest brother and co-doge Giovanni, traveled to Constantinople, where Giovanni married the niece of Basil II, Maria Argyra, and Otto received several honorific titles.

After Giovanni's sudden death in 1007, Pietro raised Otto to the dogeship with him. Pietro himself died soon afterward. He bequeathed his possessions to the poor and to the clergy and left Otto sole doge at the meager age of sixteen.

Soon after the death of his father, in 1009, Otto married Grimelda, a daughter of the newly Christian Géza of Hungary and Adelaide. Because the Chronicon Venetum of John the Deacon ends in Otto's reign, it is necessary to rely on later chronicles. According to the chronicler (and doge) Andrea Dandolo, writing from a vantage point three centuries ahead, Otto was:

Catholic in faith, calm in purity, strong in justice, eminent in religion, decorous in his manner of life, well-endowed with wealth and possessions, and so filled with all forms of virtue that he was universally considered to be the most fitting successor of his father and grandfather.

Scandal marked much of Otto's reign, as he showed a clear inclination toward nepotism with the elevation of several relatives to positions of power. In 1017 Vitale IV Candiano, Patriarch of Grado, died. Otto appointed his elder brother (Pietro's second son), Orso Orseolo, already Bishop of Torcello, to the vacant patriarchate. Otto then filled the vacant Torcello with his younger brother Vitale Orseolo. These actions lost him the support of the people, though they did not yet clamour for his removal from office. The denunciations of Poppo of Treffen, the Patriarch of Aquileia, incited the Venetians to expel Otto and the patriarch of Grado from Venice, whence they took refuge in Istria from 1022 to 1023. But in that latter year, Poppo sacked the patriarchal palace and church in Grado and the Venetians recalled Otto and Orso. In 1024, Pope John XIX confirmed Orso's right to hold Grado and confirmed the patriarchal rights of his see vis-à-vis Aquileia. However, Otto continued to use church appointments to his own personal and familial advantage and the enemies of the Orseolo in Venice, with popular support, moved to depose him in 1026. The group of rebels, led by Domenico Flabanico, imprisoned the doge and, after cutting off his beard as a sign of humiliation, exiled him, replacing him with Pietro Barbolano. Otto's brothers Orso and Vitale were deposed as well.

Otto found refuge in Constantinople, while his son Pietro sought shelter at the court of his uncle, King Stephen of Hungary. There he was well received by Constantine VIII, the uncle of his sister-in-law Maria Argyra, who repealed trade privileges previously granted to the Republic under Pietro II Orseolo. Not for nothing had Otto built up a good rapport with the emperors of Europe: the Holy Roman Emperor Conrad II likewise revoked Venetian trade privileges in response to his deposition. Stephen I of Croatia, at the instigation of Otto's son Pietro, attacked the coastal cities of Dalmatia, capturing several from Venice.