Duke John III of Denmark was a Danish nobleman born in 1525. He was the son of King Christian III of Denmark and Dorothea of Saxe-Lauenburg. John held the title of Duke of Slesvig and Holstein, regions in present-day Denmark and Germany. He was known for his efforts to promote Lutheranism and strengthen the power of the nobility. John III's reign was marked by political challenges, including conflicts with the Danish crown and rival factions. He died in 1580, leaving behind a legacy of both cultural and political significance in the region. His descendants continued to play important roles in Danish history.