| Conflict: | Siege of Nykøbing Castle |
| Place: | Nykøbing Castle, Falster, Denmark |
| Partof: | the Danish-Hanseatic War (1361–1370) |
| Date: | ? – 15 August 1368 |
| Coordinates: | 54.7717°N 11.8655°W |
| Territory: | Falster falls into Swedish rule |
| Result: | Swedish victory |
| Combatant1: | Hanseatic League |
| Commander1: | Heinrich |
| Commander2: | Henning Aldestorf |
| Units1: | Unknown |
| Units2: | Nykøbing garrison |
| Strength1: | Unknown |
| Strength2: | Unknown |
| Casualties1: | Unknown |
| Casualties2: | Unknown |
The Siege of Nykøbing or formally the Siege of Nykøbing Castle, was a siege of Nykøbing Castle, Falster between Danish and Swedish forces during the Danish-Hanseatic War (1361–1370). The Siege ended in a Swedish victory, though the Danes were able to achieve a good Instrument of Surrender.
In 1367 the Confederation of Cologne together with Holstein and Sweden attacked Denmark. Albert, King of Sweden made successful campaigns in Scania taking Lund, Ystad, and Simrishamn. From here, Albert promised to make an expedition to the Danish Isles, although he could not participate because of wounds or illness. Instead, Albert's son, Duke Henrik, would lead the combined Swedish-Hanseatic expedition.
The first goal was the island of Møn, in which the city of Stege fell. Hereafter the Swedes moved to Falster, where they began besieging Nykøbing Castle. The leader of the castle, Henning Aldestorf, defended the castle bravely, although he would surrender on 15 August. However, because of his brave actions, Duke Henrik allowed him to keep the castle if reinforcements from Valdemar IV of Denmark came before the next St. Michael's Day. If reinforcements had not come until then, then the Mecklenburgian Fikke Moltke would have taken over the castle as a subject of Albert of Sweden.
Presumably, Danish reinforcements did not come, because in 1370 Fikke Moltke acknowledged his mortgage to Albert of Sweden. With the presumed takeover of Falster, the Swedish troops moved to raid Lolland.