Legio X Gemina, was a legion of the Imperial Roman army. It was one of the four legions used by Julius Caesar in 58 BC, for his invasion of Gaul. There are still records of the X Gemina in Vienna in the beginning of the 5th century. The legion symbol was a bull. Early on in its history, the legion was called X Equestris (mounted), because Caesar once used the legionaries as cavalry.

The Roman legion, the largest military unit of the Roman army, was composed of Roman citizens serving as legionaries. During the Roman Republic the manipular legion comprised 4,200 infantry and 300 cavalry. After the Marian reforms in 107 BC the legions were formed of 5200 men and were restructured around 10 cohorts, the first cohort being double strength. This structure persisted throughout the Principate and Middle Empire, before further changes in the fourth century resulted in new formations of around 1000 men.