|Centuries:||3rd century – 4th century – 5th century|
|Decades:||270s 280s 290s – 300s – 310s 320s 330s|
|Years:||301 302 303 – 304 – 305 306 307|
|304 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1057|
|Chinese calendar||癸亥年 (Water Pig)
3000 or 2940
— to —
甲子年 (Wood Rat)
3001 or 2941
|- Vikram Samvat||360–361|
|- Shaka Samvat||226–227|
|- Kali Yuga||3405–3406|
|Iranian calendar||318 BP – 317 BP|
|Islamic calendar||328 BH – 327 BH|
|Minguo calendar||1608 before ROC
|Seleucid era||615/616 AG|
|Thai solar calendar||846–847|
Year 304 (CCCIV) was a leap year starting on Saturday (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 1057 Ab urbe condita). The denomination 304 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.
- Emperor Diocletian issues four edicts aimed at destroying Christianity; churches are to be dismantled, clergymen arrested, and their followers forced to sacrifice to pagan gods on pain of death. Those put to death include Agnes, a 12-year-old Christian girl who has refused marriage and consecrated her virginity to God. Hailed as a martyr, she will be honored as the patron saint of chastity, gardeners, rape victims and virgins.
- Constantius Chlorus defeats the Alemanni and fortifies the town of Konstanz (Germany).
- Diocletian fights a campaign against the Carpi on the Danube and becomes serious ill.
- A Triumphal Arch is built for Diocletian in Rome.
- Sixteen Kingdoms: The Wu Hu uprising establish under Liu Yuan the Han Kingdom.
- Sichuan earns its independence from China.
- October 25 – Pope Marcellinus dies at Rome after an 8-year reign. The papal throne will remain vacant until 308.
- Bunseo of Baekje, king of Baekje (Korea)
- Saint Afra (martyred by fire)
- Saint Agape, Chionia, and Irene (martyred)
- Saint Agnes (martyred)
- Saint Alban (possibly 309)
- Saint Anastasia (martyred)
- Saint Florian (martyred)
- Saint Gorgonius of Nicomedia (martyred)
- Saint Juliana of Nicomedia (martyred)
- Saint Lucie of Syracuse (martyred)
- Saint Margaret (martyred)
- Saint Pancras (martyred)
- Saint Philomena (martyred)
- Saints Theodora and Didymus (martyred)
- Saint Vincent of Saragossa (martyred)