John I was the name of a number of rulers:

• Pope John I
• King John I of France
• King John I of Castile
• King John I of Scotland
• King John I of Portugal
• King John I of Jerusalem
• King John I of Cyprus
• Emperor John I Tzimisces
• John I, Duke of Bourbon
• John I, Duke of Brabant
• John I, Duke of Brittany
• John I, Duke of Lorraine
• John I, Count of Hainaut
• John I, Count of Holland
• John I, Count of Luxemburg

John of England could also be called John I, but as there has been no second John this is unnecessary, and he is usually referred to simply as "King John".