1203 (MCCIII) was a common year starting on Wednesday of the Julian calendar, the 1203rd year of the Common Era (CE) and Anno Domini (AD) designations, the 203rd year of the 2nd millennium, the 3rd year of the 13th century, and the 4th year of the 1200s decade.
|Centuries:||12th century – 13th century – 14th century|
|Decades:||1170s 1180s 1190s – 1200s – 1210s 1220s 1230s|
|Years:||1200 1201 1202 – 1203 – 1204 1205 1206|
|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
- April 16 – Philip II of France enters Rouen, leading to the eventual unification of Normandy and France.
- April 8 – Congress in Bilino Polje, where Ban Kulin officially declared his allegiance to the Roman Catholic Church and denounced the heresy.
- June 23 – Fleet of the Fourth Crusade enters Bosphorus
- July 17 – Fourth Crusade captures Constantinople by assault; the Byzantine emperor Alexius III flees from his capital into exile.
- August 1 – Fourth Crusade elevates Alexius IV as Byzantine emperor, after the citizens of Constantinople proclaim as emperor Isaac II Angelus (Alexius IV's father).
- William de Braose, Fourth Lord of Bramber becomes the guardian of Arthur I, Duke of Brittany, and is possibly responsible for his death.
- Minamoto no Sanetomo becomes shogun of the Kamakura Shogunate