John I (15–20 November 1316), called the Posthumous, was King of France and Navarre, as the posthumous son and successor of Louis the Quarreler, for the five days he lived in 1316. Although considered a king today, his status was not recognized until chroniclers and historians in later centuries began numbering John II, thereby acknowledging John I's short reign. If his reign is recognized, it is the shortest of any French king. He is also the only person to be considered King of France since birth and, thus, the youngest person to be King of France and the only to hold the title for their entire life.
John reigned for five days under the regency of his uncle Philip the Tall, until his death on 20 November 1316. The infant King was buried in Saint Denis Basilica. He was succeeded by Philip, whose contested legitimacy led to the re-affirmation of the Salic law, which excluded women from the line of succession to the French throne.
The English translation of the royal style of King John I was "By the Grace of God, King of France and Navarre, Count of Champagne".