A centurion (/sɛnˈtjʊəriən/; Latin: centurio [kɛn̪ˈt̪ʊrioː], pl. centuriones; Ancient Greek: κεντυρίων, romanized: kentyríōn, or Ancient Greek: ἑκατόνταρχος, romanized: hekatóntarkhos) was a professional non-commissioned officer in the Roman army who commanded a group of soldiers called a centuria or "century".

The term centurion is derived from the Latin word centurio, which itself originates from centum, meaning "hundred." Initially, centurions were commanders of a unit of roughly 100 soldiers, although the exact number varied over time and by period. The concept of the centurion emerged during the early Roman Republic (509–27 BCE), when Rome's military was based on citizen-soldiers organized into centuries (centuriae), units of 100 men within the Roman legion (legio).

In the Roman infantry, the centurions commanded a centuria. During the Mid-Republic these centuries were grouped in pairs to make up a maniple, each century consisting of 30–60 men. After the "Marian reforms", a century was typically composed of around 80 men, with six such centuries forming a cohort.

Centurions were drawn from five main sources within Roman society and the military. Some were promoted from the ranks of ordinary soldiers, often after holding minor posts under the centurionate. Others, though less commonly, gained their promotion after serving in auxiliary units. The remaining three sources were the most common. One group consisted of individuals from a class whose property qualifications were below that of the equestrian order. These men received their positions in the legions as a reward for merit. Many of them were magistrates from municipal towns who, through their service, secured full Roman citizenship, making them eligible for legionary service and subsequent appointment as centurions in recognition of their prior careers.

A notable fourth category of centurions came from the equestrian order. These individuals (ordinem accepit ex equite Romano) had previously held equestrian status but either lost their wealth, failed to advance in the equestrian career (cursus honorum), or chose to resign from the order. They voluntarily accepted a centurion’s commission in the legions, often seeking a different path to distinction within the Roman military structure.

Centurions during the republican manipular system were commanders of a centuria, with two centuriae comprising a maniple. The centurions of a maniple were all subject to a hierarchy that determined their rank. During the earlier Republican manipular system, the centurions of the triarii maniples occupied the most senior position, followed by those of the principes, who were then senior to the centurions of the maniples of the hastati. Within each of the three maniples seniority increased as individuals progressed from left to right, with each maniple increasing in seniority from front to back in battle position. A further distinction existed between prior and posterior centurions within the maniple, such that the posterior centurion of the maniple of the hastati, positioned at the far front left maniple, was the most junior centurion in the legion, while the prior centurion of the maniple of the triarii, situated at the far right rear maniple, was the most senior. The prior centurion of the trarii being known as the primipilus, since from the second century the triarii were often referred to as the pilani because of their column-like formation.

Centurions were ranked hierarchically within the legion, with the ranks of centurions reflecting their seniority and responsibilities. After the Marian Reforms each legion contained 60 centurions leading their individual 60 centuriae, with 6 centuriae to a cohort and 10 cohorts to a legion, with each centurion commanding a centuria and inheriting the title of their position from the previous manipular system;

Primus Pilus (First Spear):