Year 255 (CCLV) was a common year starting on Monday of the Julian calendar. At the time, it was known in Rome as the Year of the Consulship of Valerianus and Gallienus (or, less frequently, year 1008 Ab urbe condita). The denomination 255 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.

• Sima Shi quells Guanqiu Jian and Wen Qin's rebellion.
• March 23 – Sima Shi passes away.
• Sima Zhao, Sima Shi's younger brother, inherits his brother's authority.

• Ma Jun, Chinese mechanical engineer from Cao Wei, invents the south-pointing chariot, a path-finding directional compass vehicle that uses a differential gear, not magnetics.

• January 6 – Marcellus I, bishop of Rome (d. 309)
• Dorotheus of Tyre, Syrian bishop and martyr (d. 362)
• Zhang Gui, Chinese governor of the Jin Dynasty (d. 314)
• Zuo Fen, Chinese poet of the Western Jin Dynasty (d. 300)
• Saint Helena, Roman empress who was the reputed discoverer of Christ’s cross and mother of Emperor Constantine (d. 318)

• February 23 – Guo Huai (or Boji), Chinese general
• March 16 – Guanqiu Jian, Chinese general and politician
• March 23 – Sima Shi, Chinese general and regent (b. 208)^[1]
• Fu Gu (or Lanshi), Chinese official and politician (b. 209)
• Liu Zan (or Zhengming), Chinese general (b. 183)
• Sun Luyu (or Xiaohu), Chinese princess