|Centuries:||2nd century – 3rd century – 4th century|
|Decades:||220s 230s 240s – 250s – 260s 270s 280s|
|Years:||254 255 256 – 257 – 258 259 260|
|257 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1010|
|Chinese calendar||丙子年 (Fire Rat)
2953 or 2893
— to —
丁丑年 (Fire Ox)
2954 or 2894
|Coptic calendar||−27 – −26|
|- Vikram Samvat||313–314|
|- Shaka Samvat||178–179|
|- Kali Yuga||3357–3358|
|Iranian calendar||365 BP – 364 BP|
|Islamic calendar||376 BH – 375 BH|
|Minguo calendar||1655 before ROC
|Seleucid era||568/569 AG|
|Thai solar calendar||799–800|
Year 257 (CCLVII) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Valerianus and Gallienus (or, less frequently, year 1010 Ab urbe condita). The denomination 257 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.
- Gallienus enters into a joint consulship with his father Valerianus I, having brought some order to the Danube area.
- Future emperor Aurelian defeats the Goths and brings many prisoners back to Rome.
- In Bavaria the Limes Germanicus (Upper Raetian Limes) along the river Iller is abandoned by the Romans.
- Valerian, under guardianship of Ingenuus, is established at Sirmium (Pannonia) to represent the Roman government in the troubled Illyrian provinces.
- Emperor Valerian recovers Antioch in Syria from the Persian king Shapur I.
- The Goths build a fleet on the Black Sea.
- The Goths separate into the Ostrogoths and the Visigoths.
- August 30 – Pope Sixtus II succeeds Pope Stephen I as the 24th pope.
- Valerian's persecution of Christians begins: his edict orders bishops and priests to sacrifice according to the pagan rituals, and prohibits Christians, under penalty of death, from meeting at the tombs of their deceased.
- Gregory the Illuminator, founder and patron saint of the Armenian Apostolic Church (approximate date)
- Jia Nanfeng, empress and wife of Jin Huidi (d. 300)
- August 2 – Pope Stephen I
- Zhu Yi, general of Kingdom of Wu
- Wen Qin, general of Kingdom of Wu
- Yue Lin, general of Kingdom of Wei