Year 278 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Luscinus and Papus (or, less frequently, year 476 Ab urbe condita). The denomination 278 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.
After their defeats in Greece, the Gauls move into Asia Minor. The Seleucid king Antiochus wins a major battle over the Gauls leading to his being given the title of Soter (Greek for "saviour"). The Gauls settle down to become the "Galatians" and are paid 2,000 talents annually by the Seleucid kings to keep the peace.
Antigonus concludes a peace with Antiochus who surrenders his claim to Macedonia. Thereafter Antigonus II's foreign policy is marked by friendship with the Seleucids.
Nicomedes I becomes the first ruler of Bithynia to assume the title of king. He founds the city of Nicomedia, which soon rises to great prosperity.
Chu's heartland in the modern Hubei province is overrun by the powerful state of Qin from the west under Bai Qi's leadership. The Chu government moves to the east in various temporary capitals until settling in Shouchun in 241 BC.