In ancient Rome, the plebeians or plebs were the general body of free Roman citizens who were not patricians, as determined by the census, or in other words "commoners". Both classes were hereditary.

The precise origins of the group and the term are unclear, but may be related to the Greek, plēthos, meaning masses.

In Latin, the word plebs is a singular collective noun, and its genitive is plebis. Plebeians were not a monolithic social class.

In the annalistic tradition of Livy and Dionysius, the distinction between patricians and plebeians was as old as Rome itself, instituted by Romulus' appointment of the first hundred senators, whose descendants became the patriciate. Modern hypotheses date the distinction "anywhere from the regal period to the late fifth century" BC. The 19th-century historian Barthold Georg Niebuhr believed plebeians were possibly foreigners immigrating from other parts of Italy. This hypothesis, that plebeians were racially distinct from patricians, however, is not supported by the ancient evidence. Alternatively, the patriciate may have been defined by their monopolisation of hereditary priesthoods that granted ex officio membership in the senate. Patricians also may have emerged from a nucleus of the rich religious leaders who formed themselves into a closed elite after accomplishing the expulsion of the kings.

In the early Roman Republic, there are attested 43 clan names, of which 10 are plebeian with 17 of uncertain status.

There existed an aristocracy of wealthy families in the regal period, but "a clear-cut distinction of birth does not seem to have become important before the foundation of the Republic". The literary sources hold that in the early Republic, plebeians were excluded from magistracies, religious colleges, and the Senate. However, some scholars doubt that patricians monopolised the magistracies of the early republic, as plebeian names appear in the lists of Roman magistrates back to the fifth century BC. It is likely that patricians, over the course of the first half of the fifth century, were able to close off high political office from plebeians and exclude plebeians from permanent social integration through marriage.

Plebeians were enrolled into the curiae and the tribes; they also served in the army and also in army officer roles as tribuni militum.

The Conflict of the Orders (Latin: ordo meaning "social rank") refers to a struggle by plebeians for full political rights from the patricians. According to Roman tradition, shortly after the establishment of the Republic, plebeians objected to their exclusion from power and exploitation by the patricians. The plebeians were able to achieve their political goals by a series of secessions from the city: "a combination of mutiny and a strike".