The Roman legionary (in Latin legionarius; pl.: legionarii) was a citizen soldier of the Roman army. These soldiers would conquer and defend the territories of ancient Rome during the Republic and Principate eras, alongside auxiliary and cavalry detachments. At its height, Roman Legionnaires 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 legionnaires 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 legionnaires 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. Legionnaires 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 politically prominent member of society.

The Marian reforms were putative changes to the composition and operation of the Roman army during the late Roman Republic usually attributed to Gaius Marius (a general who was consul in 107, 104–100, and 86 BC). The most important of these concerned the altering of the socio-economic background of the soldiery. Other changes were supposed to have included the introduction of the cohort; the institution of a single form of heavy infantry with uniform equipment; the universal adoption of the eagle standard; and the abolition of the citizen cavalry. It was commonly believed that Marius changed the soldiers' socio-economic background by allowing citizens without property to join the Roman army, a process called "proletarianisation". This was thought to have created a semi-professional class of soldiers motivated by land grants; these soldiers in turn became clients of their generals, who then used them to overthrow the republic.

Belief in a comprehensive scheme of reforms under Marius emerged in 1840s German scholarship, which posited that any changes in the Roman army between the times of Polybius and Marius were attributable to a single reform event. This belief was spread relatively uncritically and was accepted as largely proven by the 1850s and through much of the 20th century. There is, however, little ancient evidence for any permanent or significant change to recruitment practice in Marius' time. The occurrence of such a comprehensive reform led by Marius is no longer widely accepted by specialists; 21st-century scholars have called the reforms a "construct of modern scholarship".

Other reforms to the army's operations and equipment, said to have been implemented by Marius, are also largely rejected by scholars. Few of them have any basis in the ancient and archaeological evidence. Others are wrongly dated or misattributed. Changes in the Roman army of the late republic did occur, but appear to have happened later than at the end of the 2nd century BC. Rather, these shifts were during the Social War and following civil wars, and emerged from circumstance rather than a reformist Marian vision.

As Augustus consolidated power in 27 BC and founded the Principate, he further professionalized the Roman legionary and sought to break the legionary's dependence on his general. Under him, a legionary's term of service was raised to 25 years (before that, a legionary's average term of service was only 10 years) and pay was standardized throughout the legions. The Roman legionaries were also guaranteed a land grant or a cash payment at the end of his service, making the Roman legionary less dependent on generals for rewards after campaigns. Augustus also changed the sacramentum so that soldiers swore allegiance only to the emperor, and not to the general. Thus, Augustus managed to end the civil wars which defined the late Roman Republic and created an army that was broadly loyal to only the emperor.

Legionnaires would expand Rome's borders to include lower Britannia, Dacia, North Africa, and more through military campaigns under Augustus and future emperors.

From the reign of Septimus Severus onward, the Roman legionary gradually lost his preeminence. Though there were multiple causes for this decline, all pointed to the gradual degradation of loyalty and/or discipline. Septimus Severus, perhaps unwittingly, began this decline when he lavished his legionaries with donatives and pay increases, recognising that they were his key to becoming and staying emperor. However, this proved detrimental to the discipline of the legionaries, as they began to expect more and more rewards from their emperors. Under Caracalla, Septimus Severus's successor, all freedmen in the Roman Empire became Roman citizens, effectively erasing the distinction between auxiliaries and legionnaires. This, coinciding with the continued expansion of the Roman army, meant recruits of more dubious standards joined the legions, decreasing the quality of the Roman legionary further.