Olaf II Haakonsson (1370 – 23 August 1387) was King of Denmark as Olaf II (1376–1387) and King of Norway as Olaf IV (1380–1387). Olaf was son of King Haakon VI of Norway and the grandson of King Magnus IV of Sweden. His mother was Queen Margaret I of Denmark which made him the grandson of King Valdemar IV of Denmark. In addition to his claim on the thrones of Denmark and later Norway, he was in the direct succession line to the throne of Sweden (but for the interposition of Albert of Mecklenburg).

He became King of Denmark when only five years old and later also succeeded his father as King of Norway.

When his grandfather Valdemar IV of Denmark died, Olaf was just five years old. He was proclaimed King of Denmark by a Danehof in Slagelse the following year. His mother, Queen Margaret, was to serve as regent due to his young age. His proclamation included the title "true heir of Sweden" added at his mother's insistence since his grandfather had been king of Sweden until forced to abdicate. Olaf was hailed as king in Scania, including the towns controlled by the Hanseatic league since the Treaty of Stralsund in 1370. Queen Margaret signed a coronation charter on behalf of Olaf who was too young to rule until he came of age at fifteen. In the charter Olaf agreed to meet with the Danehof at least once a year and return properties his grandfather Valdemar IV had confiscated during his reign.

Olaf became King of Norway on his father's death in 1380. Even when Olaf reached his majority in 1385, his mother ruled through him. With his ascent to the Norwegian throne, Denmark and Norway were thus united in a personal union ruled from Denmark. Denmark and Norway would have the same king, with the exception of short interregnums, until Norway's independence in 1814.

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