Year 257 (CCLVII) was a common year starting on Thursday 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, Syria from King Shapur I of Persia.
• 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, Armenian religious leader (approximate date)
• Jia Nanfeng (or Shi), Chinese empress (d. 300)

• August 2 – Stephen I, bishop of Rome^[1]
• September 26 – Zhu Yi (or Jiwen), Chinese general
• Wen Qin (or Zhongruo), Chinese general and politician
• Zhang Changpu, Chinese concubine (b. 199)