|Centuries:||3rd century – 4th century – 5th century|
|Decades:||270s 280s 290s – 300s – 310s 320s 330s|
|Years:||300 301 302 – 303 – 304 305 306|
|303 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1056|
|Chinese calendar||壬戌年 (Water Dog)
2999 or 2939
— to —
癸亥年 (Water Pig)
3000 or 2940
|- Vikram Samvat||359–360|
|- Shaka Samvat||225–226|
|- Kali Yuga||3404–3405|
|Iranian calendar||319 BP – 318 BP|
|Islamic calendar||329 BH – 328 BH|
|Minguo calendar||1609 before ROC
|Seleucid era||614/615 AG|
|Thai solar calendar||845–846|
Year 303 (CCCIII) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Valerius and Valerius (or, less frequently, year 1056 Ab urbe condita). The denomination 303 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- Great Persecution: Emperor Diocletian launches the last major persecution of Christians in the Empire. Hierocles is said to have been the instigator of 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 20 – 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.
- Arch of Galerius dedicated in Thessaloniki.
- 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
- Anthimus of Rome
- Cao Huan, last emperor of the Kingdom of Wei (b.246)
- 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 (b.261)
- Romanus of Caesarea, martyr
- Saint Cyriacus
- Saint Devota
- Saint Pantaleon
- Victor Maurus, Christian martyr
- Vitus, Christian saint from Sicily