Legio XIII Gemina (the twin legion), was levied by Julius Caesar on 57 BC, before his attack against the tribe of the Belgae. There are still records of the XIII Gemina in Dacia (modern Romania) in the beginning of the 5th century AD. The legion symbol was a lion.

In Caesar's Gallic wars, the Thirteenth was present in the battle against the Nervians and the siege of Gergovia. When the senate refused Caesar his second consulship and civil war was about to begin, the Thirteenth was the legion that accompanied Caesar across the Rubicon river and into Italy. They remained faithful to Caesar in the civil war against Pompey, being present in the battle of Munda (45 BC) and in the African campaign. In 45 BC Caesar disbanded the legion, giving the veterans farmlands in Italy.

In 41 BC the legion was reconstituted by Augustus to deal with Pompey's sons' rebellion in Sicily. After the war against Mark Antony and the battle of Actium, the Thirteenth was reinforced with veteran legionaries from other legions and acquired the cognomen Gemina. Augustus then sent this legion to Burnum (modern Knin), in Illyricum, a Roman province in the Adriatic Sea. In 16 BC they were transferred to Emona (now Ljubljana), Pannonia, where they dealt with local rebellions.

After the disaster of the battle of the Teutoburg Forest in 9 AD, the legion was sent as reinforcements to Augusta Vindelica (Augsburg), Germania Superior to prevent further attacks from the Germanic tribes. Emperor Claudius sent them back to Pannonia around 45 AD; the legion camped at Poetovio, modern Ptuj in Slovenia.

In the year of the four emperors, XIII Gemina supported Otho, one of the defeated emperors. In 89 Domitian transferred the legion to Dacia (in Ampelum, modern Zlatna, Romania) to garrison the province from their camp in Vienna. They remained in this camp until the 5th century, according to Notitia Dignitatum, although some vexillationes participated in several foreign campaigns.

A fictionalized account some of the actions of Legio XIII Gemina during the struggle between Julius Caesar and the Optimates faction under Pompey can be seen in the joint HBO/BBC television production Rome.

• List of Roman legions and Roman legion
• livius.org account
• LEG XIII GEM, Austrian re-enactment group