| Conflict: | Siege of Lastovo |
| Date: | 1000 |
| Result: | Venetian victory |
| Combatant2: | Kingdom of Croatia Narentines |
| Commander1: | Doge Pietro II Orseolo |
| Commander2: | Unknown |
| Strength1: | Unknown |
| Strength2: | Unknown |
| Casualties1: | Unknown |
| Casualties2: | Unknown |
The siege of Lastovo in 1000 was part of the campaign of Doge Pietro II Orseolo in southern Croatia and its bloodiest armed conflict between the citizens of Lastovo island and the army of Venice. The siege resulted in a Venetian victory and Lastovo was annexed into the Venetian republic.
After the death of Croatian King Stjepan Držislav in 997, an ally of the Byzantine Empire, who ruled in the last three decades of the 10th century and was recognized by Byzantine emperor as the king of Croatia and Dalmatia, his eldest son and successor Svetoslav Suronja continued the pro-Byzantine policy, but his younger sons Krešimir and Gojslav started to organize a rebellion. With the aid of Bulgarian emperor Samuil, Krešimir and Gojslav managed to remove Svetoslav Suronja from the Croatian throne in 1000 and forced him to seek refuge in Venice.
At the same time, Doge Pietro II Orseolo prepared Venetian forces to take control over Dalmatian cities that were part of the Byzantine Empire, Croatia, and Narentines. Since the maritime city populations were mostly Latin, the majority of cities welcomed the Venetian ruler, but some of them did not. In particular, the island of Lastovo fiercely resisted the Venetian incursion.
The exact date of the siege of Lastovo is not known. According to John the Deacon, Orseolo's chronicler, the doge attended the Holy Mass on Ascension Day (9 May 1000) in Venice. Then he led his fleet towards Grado to visit the patriarch and on 11 May he sailed southwards.
As the fleet reached Lastovo, the residents of the town did not welcome it, but waited behind the city walls, ready to resist. Orseolo ordered them to surrender but they refused, so he started to attack the town. His troops besieged it, trying to break the town gate as well as to seize the towers of the wall. They succeeded in capturing the tower with the water cistern, which was a big setback for the remaining defenders. Finally, they were forced to surrender and to lay down their weapons. The doge spared their lives, but gave the order to his soldiers to destroy the defensive walls and to burn down the town. The citizens were moved away to the other side of the island, where they built a new settlement.
Pietro II Orseolo subjected Lastovo to Venetian authority. Then, he moved further with his fleet towards Dubrovnik, whose dignitaries showed loyalty to him, and returned subsequently to Venice. He started to carry the title of Dux Dalmatianorum (Duke of the Dalmatians).
For the next few decades, the island of Lastovo remained a Venetian possession. It came back and stayed under Croatian rule during the reign of King Stjepan I (ruled 1030-1058) and his successors. In 1252 the residents of Lastovo decided to join the Community of Dubrovnik (ital. Ragusa), (later Republic of Dubrovnik).