The Roman legionary (in Latin legionarius, plural legionarii) was a professional heavy infantryman of the Roman army after the Marian reforms. These soldiers would conquer and defend the territories of ancient Rome during the late Republic and Principate eras, alongside auxiliary and cavalry detachments. At its height, Roman legionaries were viewed as the foremost fighting force in the Roman world, with commentators such as Vegetius praising their fighting effectiveness centuries after the classical Roman legionary disappeared.

Roman legionaries were recruited from Roman citizens under age 45. They were first predominantly made up of recruits from Roman Italy, but more were recruited from the provinces as time went on. As legionaries moved into newly conquered provinces, they helped Romanize the native population and helped integrate the disparate regions of the Roman Empire into one polity. They enlisted in a legion for 25 years of service, a change from the early practice of enlisting only for a campaign. Legionaries were expected to fight, but they also built much of the infrastructure of the Roman Empire and served as a policing force in the provinces. They built large public works projects, such as walls, bridges, and roads. The legionary's last five years of service were on lighter duties. Once retired, a Roman legionary received a parcel of land or its equivalent in money and often became a prominent member of society.