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|303 by topic|
|Ab urbe condita||1056|
|Balinese saka calendar||224–225|
|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||224–225|
|- Kali Yuga||3403–3404|
|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|
429 or 48 or −724
— to —
430 or 49 or −723
Year 303 (CCCIII) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. It was known in the Roman Empire 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.
- The Arch of Galerius is dedicated in Thessaloniki.
- Etchmiadzin Cathedral is completed by Gregory the Illuminator and Tiridates III, king of Armenia.
- January 6 – baptization of Tiridates III of Armenia.
- Magnentius, Roman usurper (d. 353)
- Wang Xizhi, Chinese calligrapher (d. 361)
- Xun Guan, Chinese female warrior
- 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
- Saint Cyriacus
- Saint Devota
- Erasmus of Formiae
- Expeditus, Roman commander and martyr
- Felix and Adauctus, Christian martyrs
- Saint Fermin, Roman Catholic saint
- Lu Ji, Chinese writer and literary critic (b.261)
- Saint Pantaleon
- Romanus of Caesarea, martyr
- Victor Maurus, Christian martyr
- Vitus, Christian saint from Sicily
- Vagi, David (2016). Coinage and History of the Roman Empire. Routledge. p. 476. ISBN 978-1-135-97125-0.
- "Lu Ji's (261–303) Essay on Literature dated 1544 and 1547". www.metmuseum.org. Retrieved January 5, 2020.