203 (CCIII) was a common year starting on Saturday of the Julian calendar, the 203rd year of the Common Era (CE) and Anno Domini (AD) designations, the 203rd year of the 1st millennium, the 3rd year of the 3rd century, and the 4th year of the 200s decade. As of the start of 203, the Gregorian calendar was in agreement with the Julian calendar, which was the dominant calendar of the time.

• Emperor Septimius Severus rebuilds Byzantium.
• Gaius Fulvius Plautianus and Publius Septimius Geta become Roman Consuls.
• An arch dedicated to Severus is put up near the Forum.
• The Portico of Octavia is rebuilt.

• Reign in India of Vijaya, Satavahana king of the Andhra.
• The Andhra empire is being broken apart into smaller and smaller independent sections.

• Origen of Alexandria replaces Clement as the head of the Christian school in Alexandria.

• November – Mahavatar Babaji in Parangipettai, Tamil Nadu, India
• Probable birth of Roman Emperor Marcus Aurelius Antoninus, better known as Elagabalus (d. 222)
• Cao Rui, emperor of the Kingdom of Wei (approximate date) (d. 239)
• Zhuge Ke, son of Zhuge Jin, officer of the Wu Kingdom (d. 253)

• Perpetua, Christian saint (martyred)
• Felicitas, Christian saint (martyred)
• Ling Cao, general under Sun Ce and father of Ling Tong (killed by pirate Gan Ning)
• Wu Jing