Year 340 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Torquatus and Mus (or, less frequently, year 414 Ab urbe condita). The denomination 340 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.
Pixodarus, the youngest of the three sons of King Hecatomnus of Caria, gains possession of the satrapy of Caria by expelling his sister Ada, the widow and successor of her brother Idrieus.
Philip II fails in his siege of Byzantium and is forced to respond to attacks by the Scythians near the mouth of the Danube. His son, Alexander is regent while his father fights against Byzantium and the Scythians.
The Athenians give Demosthenes a public vote of thanks after Philip's unsuccessful siege of Byzantium.