The Roman legion (Latin: legiō, Latin: [ˈɫɛɡioː]) was the largest military unit of the Roman army, composed of Roman citizens serving as legionaries. During the Roman Republic the manipular legion comprised 4,200 infantry and 300 cavalry. In late Republican times the legions were formed of 5,200 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 1,000 men.

The size of a typical legion varied throughout the history of ancient Rome, with complements ranging from 4,200 legionaries and 300 equites (drawn from the wealthier classes – in early Rome all troops provided their own equipment) in the Republic, to 5,500 in the Imperial period, when most legions were led by a Roman Imperial Legate.

A legion had 4,800 legionaries (in 10 cohorts of 6 centuries of 80 legionaries) from the late republic to the time of Julius Caesar. It expanded to 5,280 men plus 120 auxiliaries in the Imperial period (split into 10 cohorts, nine of 480 men each, with the first cohort being almost double-strength at 800 men). These are typical field strengths while "paper strength" was slightly higher (e.g. 600 and 1,200 respectively for Imperial cohorts).

In the early Roman Kingdom the term legion may have meant the entire Roman army, but sources on this period are few and unreliable. The subsequent organisation of legions varied greatly over time but legions were typically composed of around five thousand soldiers. During much of the republican era, a legion was divided into three lines, each of ten maniples. In the late Republic and much of the imperial period (from about 100 BC), a legion was divided into ten cohorts, each of six (or five) centuries. Legions also included a small ala, or cavalry unit. By the third century AD, the legion was a much smaller unit of about 1,000 to 1,500 men, and there were more of them. In the fourth century AD, East Roman border guard legions (limitanei) may have become even smaller. In terms of organization and function, the republican era legion may have been influenced by the ancient Greek and Macedonian phalanx.

For most of the Roman Imperial period, the legions formed the Roman army's elite heavy infantry, recruited exclusively from Roman citizens, while the remainder of the army consisted of auxiliaries, who provided additional infantry and the vast majority of the Roman cavalry (provincials who aspired to Roman citizenship gained it when honourably discharged from the auxiliaries). The Roman army, for most of the Imperial period, consisted mostly of auxiliaries rather than legions.

Many of the legions founded before 40 BC were still active until at least the fifth century, notably Legio V Macedonica, which was founded by Augustus in 43 BC and was in Egypt in the seventh century during the Islamic conquest of Egypt.

On the other hand, Legio XVII ("Seventeenth Legion"), Legio XVIII ("Eighteenth Legion") and Legio XIX ("Nineteenth Legion"), founded by Augustus around 41 BC, were destroyed by a Germanic alliance led by Arminius in the Varian Disaster (September 9, AD 9) and never raised again by the Romans thereafter.

Quintili Vare, legiones redde! (Quintilius Varus, give me back my legions!)