Duke John I of Schleswig was a nobleman who ruled the duchy of Schleswig in the late 14th century. He was born around 1350 and became Duke in 1376, following the death of his father, Duke Valdemar IV. John I's reign was marked by efforts to strengthen the duchy's position in the region and maintain its independence. During his rule, John I faced challenges from neighboring territories, including Denmark and Holstein. He worked to secure alliances and manage conflicts, but his reign was relatively short, ending with his death in 1386. His legacy continued through his descendants, who played significant roles in the history of Schleswig.