Year 343 (CCCXLIII) was a common year starting on Saturday of the Julian calendar. At the time, it was known as the Year of the Consulship of Memmius and Romulus (or, less frequently, year 1096 Ab urbe condita). The denomination 343 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.

• The Western Roman Emperor Constans I is in Britain, possibly in a military campaign against the Picts and Scots (the last visit to Britain by a legitimate emperor).^[1]
• The Eastern Roman Emperor Constantius II campaigns in Adiabene, a vassal kingdom of Armenia (Persian Empire).

• Pope Julius I tries to unite the Western bishops against Arianism by convoking the Council of Serdica (later Sofia), which acknowledges the pope's supreme authority and grants him the right to judge cases involving the legal possession of episcopal sees, but only Western and Egyptian bishops attend, and Arianism remains strong.

• Fu Deng, Chinese emperor of the Di state Former Qin (d. 394)
• Jin Mudi, Chinese emperor of the Eastern Jin Dynasty (d. 361)
• Xiao Wenshou (or Xiaoyi), Chinese empress dowager (d. 423)
• Xie Xuan, Chinese general of the Jin Dynasty (d. 388)

• December 6 – Saint Nicholas (or Santa Claus) (b. 270)^[2]
• Li Shou (or Wukao), Chinese emperor of Cheng Han (b. 300)