|Centuries:||3rd century – 4th century – 5th century|
|Decades:||270s 280s 290s – 300s – 310s 320s 330s|
|Years:||303 304 305 – 306 – 307 308 309|
|306 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1059|
|Bahá'í calendar||−1538 – −1537|
|English Regnal year||N/A|
|Chinese calendar||乙丑年 (Wood Ox)
3002 or 2942
— to —
丙寅年 (Fire Tiger)
3003 or 2943
|- Vikram Samvat||362–363|
|- Shaka Samvat||228–229|
|- Kali Yuga||3407–3408|
|Igbo calendar||−694 – −693|
|Iranian calendar||316 BP – 315 BP|
|Islamic calendar||326 BH – 325 BH|
|Minguo calendar||1606 before ROC
|Thai solar calendar||849|
|Wikimedia Commons has media related to 306.|
Year 306 (CCCVI) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantius and Valerius (or, less frequently, year 1059 Ab urbe condita). The denomination 306 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.
- Constantius Chlorus undertakes a punitive expedition against the Picts beyond the repaired Hadrian's Wall. His son Constantine I joins him on campaign and they win a brilliant victory.
- July 25 – Constantius Chlorus dies outside Eboracum (modern York). Constantine I, age 26, is declared Augustus (emperor) by his troops and awaits recognition by Emperor Galerius.
- Galerius grants Constantine I the title Caesar and elevates Severus II to co-emperor of the Western Roman Empire.
- October 28 – Maxentius, son of the former Western Emperor Maximian, joins a revolt by the Praetorian Guard in Rome, and is proclaimed Emperor.
- Southern Italy bitter over the subjection to taxation, supports Maxentius, as does Africa, Corsica, Sardinia and Sicily.
- Galerius sends Severus II to suppress the rebellion in Rome. The soldiers desert him and Severus flees to Ravenna.
- The Baths of Diocletian are dedicated, the thermae become the largest imperial baths in Rome.
- The Franks cross the Rhine, but are repulsed by Constantine I.
- The Santa Maria Maior de Lisboa is built in Lisbon.
- The Council of Elvira declares that killing through a spell is a sin and the work of the devil.
- Saint Metrophanes becomes bishop of Byzantium.
- Christianity is established in Britain. British bishops participate in the councils of Arles (314), Nicaea (325) and Arminum (349).
- July 25- Constantius Chlorus, Roman Emperor
- August 25- Saint Maginus, Christian hermit and martyr from Tarragona, Catalonia; beheaded.
- Saint Theodore of Amasea, Christian martyr
- Sima Ying, crown prince of the Jin Dynasty (b. 279)