Year 1203 (MCCIII) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. It was also the first year to have all digits different from each other since 1098.
|Ab urbe condita||1956|
|Balinese saka calendar||1124–1125|
|English Regnal year||4 Joh. 1 – 5 Joh. 1|
|Chinese calendar||壬戌年 (Water Dog)|
3899 or 3839
— to —
癸亥年 (Water Pig)
3900 or 3840
|- Vikram Samvat||1259–1260|
|- Shaka Samvat||1124–1125|
|- Kali Yuga||4303–4304|
|Japanese calendar||Kennin 3|
|Minguo calendar||709 before ROC|
|Thai solar calendar||1745–1746|
1329 or 948 or 176
— to —
1330 or 949 or 177
|Wikimedia Commons has media related to 1203.|
- April 16 – Philip II of France enters Rouen, leading to the eventual unification of Normandy and France.
- William de Braose, 4th Lord of Bramber, becomes the guardian at Rouen of the imprisoned Arthur I, Duke of Brittany, designated heir to the throne of England, who is not known to be alive after April and is probably murdered by or at the orders of his uncle, Richard I of England.
- The troops of the Fourth Crusade reach the Byzantine heartland:
- June 23 – The Fleet of the crusaders enters the Bosphorus.
- July 17 – The armies of the Fourth Crusade capture Constantinople by assault; the Byzantine emperor Alexios III Angelos flees from his capital into exile.
- August 1 – The Fourth Crusade elevates Alexios IV Angelos as Byzantine emperor, after the citizens of Constantinople proclaim as emperor Isaac II Angelos (Alexius IV's father).
- The House of Burke is founded in Ireland.
- Battle of Basiani: The Georgians defeat a Muslim coalition.
- The Almohads begin the conquest of the Balearic Islands.
- The Oeselians ravage Danish Scania. The returning pirates later skirmish with the German settlers of Riga, near Visby in Gotland.
- First evidence that the Temple in London is extending loans to the king of England. The sums remain relatively small, but are often used for critical operations, such as the ransoming of the king’s soldiers captured by the French.
- January 21 – Agnes II, Abbess of Quedlinburg (b. 1139)
- April 6 – William of Æbelholt, Danish churchman (b. 1125)
- November 4 – Dirk VII, Count of Holland
- May – Dafydd ab Owain Gwynedd, King of Gwynedd
- date unknown – Alexios Palaiologos, Byzantine heir apparent
- probable – Arthur I, Duke of Brittany (b. 1187)
- Ferris, Eleanor (1902). "The Financial Relations of the Knights Templars to the English Crown". American Historical Review. 8 (1).