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Year 1217 (MCCXVII) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar.
- Mukhali returns to Genghis Khan's camp in Mongolia, and receives the hereditary title of prince, a golden seal, and a white standard with nine tails and a black crescent in the middle. He is appointed commander in chief of operations in North China.
- The Fifth Crusade reaches the Holy Land.
- April 9 – Peter II of Courtenay is crowned emperor of the Latin Empire of Constantinople at Rome, by Pope Honorius III.
- May 20 – First Barons' War in England: Occupying French forces are defeated at the Battle of Lincoln, by English royal troops led by William Marshal, Earl of Pembroke, and survivors are forced to flee south.
- June 6 – King Henry I of Castile dies aged 13, from the fall of a roof tile in Palencia, an event which his regent, Álvaro Núñez de Lara, attempts to conceal. Henry's sister Berengaria succeeds to the throne.
- June – Haakon Haakonsson becomes King Haakon IV of Norway, following the death of Inge II, and largely ends the civil war era in Norway, reigning until 1263.
- August 24 – First Barons' War: In the Battle of Sandwich in the English Channel, English forces destroy the French, and the French mercenary Eustace the Monk is captured and beheaded.
- August 31 – Ferdinand becomes King of Castile, upon the abdication of his mother Berengaria.
- September 12 – The First Barons' War in England is ended by the Treaty of Kingston upon Thames; The French and Scots are to leave England, and an amnesty is granted to rebels.
- September 20 – The Treaty of Lambeth is signed, ratifying the Treaty of Kingston.
- September 21 – Livonian Crusade: The Livonian Brothers of the Sword and allied Livs and Letts defeat the Estonian army in the Battle of St. Matthew's Day, and kill their leader Lembitu.
- October 18 – Fifth Crusade, Reconquista: Alcácer do Sal is reconquered by the troops of King Afonso II of Portugal.
- November – In the Kingdom of Castile, former regent Álvaro Núñez de Lara is captured, and forced to relinquish all his castles.
- Stefan Nemanjić is elevated to be the first King of the Serbian lands by Pope Honorius III, and crowned by Stefan's brother, Archimandrite Sava, in Žiča.
- A decree made in England establishes that only Englishmen can be clergy of Ireland.