303

303 (CCCIII) was a common year starting on Friday of the Julian calendar, the 303rd year of the Common Era (CE) and Anno Domini (AD) designations, the 303rd year of the 1st millennium, the 3rd year of the 4th century, and the 4th year of the 300s decade. As of the start of 303, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.

• Great Persecution: Emperor Diocletian launches the last major persecution of Christians in the Empire. Hierocles is said to have started the fierce persecution of the Christians under Galerius. They are forbidden to worship in groups, and thousands of them are killed in the next 10 years.
• February 24 – Galerius publishes his edict that begins the persecution of Christians in his portion of the Empire.
• September 25 – On a voyage preaching the gospel, Saint Fermin of Pamplona is beheaded in Amiens, France.
• November 28 – Diocletian makes a visit to Rome. The Augusti and the Caesars are united for the first time to celebrate the 20th anniversary of Diocletian's accession.
• The Triumphal Arch of Maximian is erected in Salonica.

• Sassanid Shah Hormizd II succeeds Narseh.

• In Mexico the civilization of Teotihuacan flourishes.

• March 4 – Martyrdom of Saint Adrian of Nicomedia.

• Magnentius, Roman usurper (d. 353)
• Wang Xizhi, Chinese calligrapher (d. 361)

• April 23 – Saint George, soldier of the Roman Empire
• September 25 – Saint Fermin, Roman Catholic saint
• Acacius of Byzantium, centurion in the Roman army and martyr
• Adrian of Nicomedia, Christian saint
• Anthimus of Rome
• Cao Huan, last emperor of the Kingdom of Wei
• Cessianus, Roman Catholic saint and martyr
• Crescentinus, patron saint of Urbino
• Erasmus of Formiae
• Expeditus, Roman commander and martyr
• Felix and Adauctus, Christian martyrs
• Lu Ji, Chinese writer and literary critic
• Romanus of Caesarea, martyr
• Saint Cyriacus
• Saint Devota
• Saint Pantaleon
• Victor Maurus, Christian martyr
• Vitus, Christian saint from Sicily