In ancient Roman religion and mythology, Mars (Latin: Mārs, pronounced [maːrs]) is the god of war and also an agricultural guardian, a combination characteristic of early Rome. He is the son of Jupiter and Juno, and was pre-eminent among the Roman army's military gods. Most of his festivals were held in March, the month named for him (Latin Martius), and in October, the months which traditionally began and ended the season for both military campaigning and farming.

Under the influence of Greek culture, Mars was identified with the Greek god Ares, whose myths were reinterpreted in Roman literature and art under the name of Mars. The character and dignity of Mars differs in fundamental ways from that of his Greek counterpart, who is often treated with contempt and revulsion in Greek literature. Mars's altar in the Campus Martius, the area of Rome that took its name from him, was supposed to have been dedicated by Numa, the peace-loving semi-legendary second king of Rome; in Republican times it was a focus of electoral activities. Augustus shifted the focus of Mars's cult to within the pomerium (Rome's ritual boundary), and built a temple to Mars Ultor as a key religious feature of his new forum.

Unlike Ares, who was viewed primarily as a destructive and destabilizing force, Mars represented military power as a way to secure peace, and was a father (pater) of the Roman people. In Rome's mythic genealogy and founding, Mars fathered Romulus and Remus through his rape of Rhea Silvia. The wolf was the sacred animal of Mars, with the she-wolf nursing the two founders as children. His love affair with Venus symbolically reconciled two different traditions of Rome's founding; Venus was the divine mother of the hero Aeneas, credited by Vergil as an earlier founder of Rome.

The word Mārs (genitive Mārtis), which in Old Latin and poetic usage also appears as Māvors (Māvortis), is cognate with Oscan Māmers (Māmertos). The oldest recorded Latin form, Mamart-, is likely of foreign origin. It has been explained as deriving from Maris, the name of an Etruscan child-god, though this is not universally agreed upon. Scholars have varying views on whether the two gods are related, and if so how. Latin adjectives from the name of Mars are martius and martialis, from which derive English "martial" (as in "martial arts" or "martial law") and personal names such as "Marcus", "Mark" and "Martin".

Mars may ultimately be a thematic reflex of the Proto-Indo-European god Perkwunos, having originally a thunderer character.

Like Ares who was the son of Zeus and Hera, Mars is usually considered to be the son of Jupiter and Juno. In Ovid's version of Mars's origin, he was the son of Juno alone. Jupiter had usurped the role of mother when he gave birth to Minerva directly from his forehead (or mind) without a female partner. Juno sought the advice of the goddess Flora on how in turn to produce a child without male intervention. Flora obtained a magic flower (Latin flos, plural flores, a masculine word) and tested it on a heifer who became fecund at once. Flora ritually plucked a flower, using her thumb, touched Juno's belly, and impregnated her. Juno withdrew to Thrace and the shore of Marmara for the birth.

Ovid tells this story in the Fasti, his long-form poetic work on the Roman calendar. It may explain why the Matronalia, a festival celebrated by married women in honor of Juno as a goddess of childbirth, occurred on the first day of Mars's month, which is also marked on a calendar from late antiquity as the birthday of Mars. In the earliest Roman calendar, March was the first month, and the god would have been born with the new year. Ovid is the only source for the story. He may be presenting a literary myth of his own invention, or an otherwise unknown archaic Italic tradition; either way, in choosing to include the story, he emphasizes that Mars was connected to plant life and was not alienated from female nurture.

The consort of Mars was Nerio or Neriene, meaning "Valor". She represents the vital force (vis), power (potentia) and majesty (maiestas) of Mars. Her name was regarded as Sabine in origin and is equivalent to Latin virtus, "manly virtue" (from vir, "man"). In the early 3rd century BCE, the comic playwright Plautus has a reference to Mars greeting Nerio, his wife. A source from late antiquity says that Mars and Neriene were celebrated together at a festival held on March 23. In the later Roman Empire, Neriene came to be identified with Minerva.