Virtus (Classical Latin: [ˈwɪrtuːs̠]) was a specific virtue in ancient Rome that carried connotations of valor, masculinity, excellence, courage, character, and worth, all perceived as masculine strengths. It was thus a frequently stated virtue of Roman emperors, and was personified as the deity Virtus.

The origins of the word virtus can be traced back to the Latin word vir, 'man'. The common list of attributes associated with virtus were considered to be particularly masculine strengths. From the early to the later days of the Roman Empire, there appears to have been a development in how the concept was understood.

Originally virtus described specifically martial courage, but it eventually grew to be used to describe a range of Roman virtues. It was often divided into different qualities including prudentia (practical wisdom), iustitia (justice), temperantia (temperance, self-control), and fortitudo (courage). This division of virtue as a whole into cardinal virtues is an ongoing project of positive psychology or, in philosophy, virtue ethics, following a tradition originating in Plato's Republic and Aristotle's Nicomachean Ethics. It implies a link between virtus and the Greek concept of arete.

At one time virtus extended to include a wide range of meanings that covered one general ethical ideal. The use of the word grew and shifted to fit evolving ideas of what manliness meant. Once, virtus meant primarily that a man was a brave warrior, but it came also to mean that he was a good man, someone who did the right thing. During the time of the decline of the Roman elite, the Roman upper class no longer thought of themselves as unmanly if they did not serve in the military.

Virtus came from an aristocratic tradition in which it described a specific type of public conduct. It was mainly applicable to those in the cursus honorum, certainly by the late republic at least. It was not a "private" virtue in the way that modern people might consider it. Valor, courage, and manliness were not things to be pursued in the private sphere of the individual or the individual's private concerns. There could be no virtue in exploiting one's manliness in the pursuit of personal wealth, for example. Virtus was exercised in the pursuit of gloria for the benefit of the res publica resulting in the winning of eternal memoria. According to D.C. Earl, "Outside the service of the res publica there can be no magistratus and therefore, strictly speaking, no gloria, no nobilitas, no virtus."

The nobility of virtus lay not only in one's personal acta but also those of one's ancestors. However Cicero, a novus homo, asserted that virtus was a virtue particularly suited to the new man just as nobilitas was suited to the noble. Cicero argued that just as young men from noble families won the favor of the people so too should the novus homo earn the favor of the people with his virtus. He asserted that virtus, and not one's family history, should decide a man's worthiness. Because virtus is something that a man earns himself, not something that is given to him by his family, it is a better measure of a man's ability. Cicero's goal was not to impugn the noble class but widen it to include men who had earned their positions by merit.

The term virtus was used quite significantly by the historian Sallust, a contemporary of Cicero. Sallust asserted that virtus did not rightfully belong to the nobilitas as a result of their family background but specifically to the novus homo through the exercise of ingenium (talent, also means sharpness of mind, sagacity, foresight, and character). For Sallust and Cicero alike, virtus comes from winning glory through illustrious deeds (egregia facinora) and the observance of right conduct through bonae artes.

Virtus was not universally applicable—typically only adult male Roman citizens could be thought of as possessing virtus.