340s

The 340s decade ran from January 1, 340, to December 31, 349.

• Constantinople, capital of Emperor Constantius II, becomes the largest city in the world, taking the lead from Rome, capital of his brother Constans I.
• Constantine II, emperor of the western part of the Roman Empire (Britain, Gaul, the Rhine provinces and Iberia), crosses the Alps and attacks the army of his brother Constans I, emperor of the central part of the Roman Empire (Upper Danube, Italy and middle Africa). They clash at Aquileia in northern Italy. Constantine is killed in a skirmish by an ambush of Constans' troops.
• Constans is left sole ruler of the Western part of the Roman Empire, with his other brother, Constantius II, emperor of the Eastern portion.

• Acacius succeeds Eusebius as bishop in the see of Caesarea.
• Wulfila evangelizes among the Goths for 7 years.

• Emperor Constans I bans pagan sacrifices and magic rituals, under penalty of death.
• Constans I begins a successful campaign against the Franks.

• Samudragupta of the Gupta Empire, during a decade, extends his kingdom and his influence. A pillar found at Allahabad sings his praises.

• The Council of Encaenia is held in Antioch.
• Paul I is restored as Patriarch of Constantinople.
• Thousands of Christians are executed at Seleucia in Mesopotamia.
• Coptic Christianity is introduced into Ethiopia by the Syrian apostle Frumentius. He and his colleague Aedesius ware captured by Ethiopians a year or two ago, and have become civil servants at the Aksumite court of King Ezana. Ezana is impressed with Frumentius' teachings and converts to Christianity. Frumentius becomes the first Bishop of Axum and encourages the Christian merchants present in the country to practise their faith openly.

• Emperor Constans I campaigns in Britain against the Picts.
• Constans I campaigns victoriously against the Franks.

• A large earthquake strikes Cyprus.
• Goguryeo is invaded by Murong Huang of the Xianbei.
• Jin Kangdi succeeds his brother Jin Chengdi as emperor of China.

• Paul I, Patriarch of Constantinople, is deposed and replaced by Macedonius I.
• February 15 – The original Hagia Sophia is dedicated in Constantinople.

• 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).
• 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, 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.

• The Eastern Roman Emperor Constantius II campaigns in eastern Mesopotamia, against the Sassanid Persians.
• Battle of Singara: The Roman army under Constantius wins a close victory, at the strongly fortified city of Singara (Mesopotamia). His enemy, King Shapur II, is forced to lift the siege, and withdraw the Persian army.
• Shapur II, for the second time, besieges the Roman fortress of Nisibis in eastern Mesopotamia, but is repulsed by forces under General Lucilianus.

• Jin Mudi, age 1, succeeds his father Jin Kangdi as emperor of China. His mother, Empress Dowager Chu, becomes the ruling authority at court, and serves as regent.
• Gye becomes king of the Korean kingdom of Baekje.

• The making of a detail of Admonitions of the Imperial Instructress to Court Ladies (attributed to Gu Kaizhi and being from the Six Dynasties period) begins (approximate year) and is completed in 406. It is now kept at the British Museum, London.

• Bishop Eustorgius I brings relics of the Three Magi from Constantinople to Milan, according to a 12th century legend.

• Merchant Knai Thomman and 400 followers visit the Malabar Coast in Kerala (India), and assist the church there.
• The Kadamba Dynasty is founded by Mayurasharma.

• Constans orders the Basilica di Santa Tecla to be constructed in Milan.

• In Korea, the Buyeo Kingdom is absorbed by Goguryeo.
• Geunchogo becomes king of the Korean kingdom of Baekje.

• Emperor Constans I uses his influence to secure the return of Athanasius. He is restored as Patriarch of Alexandria, and documents are compiled relating to his expulsion, under the title Apology Against the Arians.
• Macedonius I, Patriarch of Constantinople, is deposed again by Paul I.
• Julius Firmicus Maternus writes De erroribus profanarum religionum.
• The Visigoths are converted to Arianism by Wulfila.

• Li Shi, ruler of Cheng Han, fails in his attempt to halt a Jin expedition under Huan Wen. He flees to the capital Chengdu and surrenders his forces. Emperor Jin Mudi spares his life and makes him a marquess.

• Council of Sardica: An attempt is made to resolve the Arian controversy, and ground rules for bishops are laid down.
• The Council of Philippopolis is held as the result of Eastern bishops leaving the Council of Sardica. In Philippopolis (Bulgaria), they excommunicate Pope Julius, and as a result, the Arian controversy is perpetuated.

• Wulfila escapes religious persecution by the Gothic chieftain Athanaric, obtains permission from Constantius II to migrate with his flock of converts to Moesia, and settles near Nicopolis ad Istrum (Bulgaria).

• In Persia, women are enrolled in the army to perform auxiliary services.
• In India, Samudragupta of the Gupta Empire defeats Rudrasena in battle.

• Shi Hu, emperor of the Jie state Later Zhao since 334, dies. The state plunges into turmoil with his sons Shi Shi, Shi Zun, Shi Jian and Shi Zhi plotting against each other and holding the emperorship in rapid succession, before ethnic Han Ran Min establishes the short-lived Ran Wei dynasty in 350, bringing the Later Zhao dynasty to an end in 351 before it is itself conquered and divided by the Former Yan and Former Qin dynasties in 351.
• Shi Shi, youngest of Shi Hui's sons, reigns for 33 days before being deposed and executed at age 10 by Shi Zun. After a brief reign of 183 days, Emperor Shi Zun and his mother Empress Zheng Yingtao are executed; his brother Shi Jian succeeds him, only to be toppled after 103 days in early 350 by Shi Zhi,