The Ara Pacis Augustae (Latin, "Altar of Augustan Peace"; commonly shortened to Ara Pacis) is an altar in Rome dedicated to the Pax Romana. The monument was commissioned by the Roman Senate on July 4, 13 BC to honour the return of Augustus to Rome after three years in Hispania and Gaul and consecrated on January 30, 9 BC. Originally located on the northern outskirts of Rome, a Roman mile from the boundary of the pomerium on the west side of the Via Flaminia, the Ara Pacis stood in the northeastern corner of the Campus Martius, the former flood plain of the Tiber River and gradually became buried under 4 metres (13 ft) of silt deposits. It was reassembled in its current location, now the Museum of the Ara Pacis, in 1938, turned 90° counterclockwise from its original orientation so that the original western side now faces south.

The altar reflects the Augustan vision of Roman civil religion. The lower register of its frieze depicts agricultural work meant to communicate the abundance and prosperity of the Roman Peace (Latin: Pax Romana). The monument as a whole serves a dual civic ritual and propaganda function for Augustus and his regime, easing notions of autocracy and dynastic succession that might otherwise be unpalatable to traditional Roman culture.

The monument consists of a traditional open-air altar at its centre surrounded by precinct walls which are pierced on the eastern and western ends (so called today because of the modern layout) by openings and elaborately and finely sculpted entirely in Luna marble.

Within the enclosing precinct walls, the altar itself was carved with images illustrating the lex aria, the law governing the ritual performed at the altar. The sacrificial procession depicts animals being led to sacrifice by figures carved in a Republican style similar to the so-called "Altar of Domitius Ahenobarbus", in sharp contrast with the style on the exterior of the precinct walls. What remains of the altar is otherwise fragmentary, but it appears to have been largely functional with less emphasis on art and decoration.

The interior of the precinct walls are carved with bucrania, ox skulls, from which carved garlands hang. The garlands bear fruits from various types of plants, all displayed on a single garland as allegorical representations of plenty and abundance. The bucrania in turn evoke the idea of sacrificial piety, appropriate motifs for the interior of the altar precinct. The lower register of the interior walls imitate the appearance of traditionally wooden altar precincts, which were meant to bring to mind other such altars in Rome and the tradition of constructing altars at the boundary of the city's pomerium.

The exterior walls of the Ara Pacis are divided between allegorical and pseudo-historical relief panels on the upper register while the lower register comprises scenes of nature: harmonic, intertwined vines that contain wildlife and connote nature under control. The upper register of the northern and southern walls depict scenes of the emperor, his family, and members of the regime in the act of processing to or performing a sacrifice. Various figures in togas are shown with their heads covered (capite velato), signifying their role as both priests and sacrificiants. Other figures wear laurel crowns, traditional Roman symbols of victory. Members of individual priestly colleges are depicted in traditional garb appropriate to their office, while lictors can be identified by their iconographic fasces. Women and children are also included among the procession; the depiction of children in Roman sculpture would have been novel at the time of the Altar's construction, evoking themes of moral and familial piety, as well as easing concerns over dynastic intentions while simultaneously introducing potential heirs to the public eye. However, despite the emphasis on family values, the anonymity of many of the women depicted enforces the male retention of power within this new age of peace.

The western and eastern walls are both pierced by entryways to the altar, although the interior would only have been accessed by a stairway on the western side. The entryways were flanked by panels depicting allegorical or mythological scenes evocative of peace, piety and tradition. On the eastern wall, panels depicted the seated figures of Roma and Pax, while the western side depicts the discovery of the twins and she-wolf and the sacrifice of a figure traditionally identified as Aeneas, but increasingly believed to be Rome's second king, Numa Pompilius. The identity of these various figures has been a point of some controversy over the years, relying heavily on interpretation of fragmentary remains, discussed below.

The sculpture of the Ara Pacis is primarily symbolic rather than decorative, and its iconography has several levels of significance. Studies of the Ara Pacis and similar public Roman monuments traditionally address the potent political symbolism of their decorative programs, and their emphasis and promulgation of dynastic and other imperial policies; they are usually studied as a form of imperial propaganda. The Ara Pacis is seen to embody without conscious effort the deep-rooted ideological connections among cosmic sovereignty, military force, and fertility that were first outlined by Georges Dumézil, connections which are attested in early Roman culture and more broadly in the substructure of Indo-European culture at large. Peter Holliday suggested that the Altar's imagery of the Golden Age, usually discussed as mere poetic allusion, appealed to a significant component of the Roman populace. The program of the Ara Pacis addressed this group's very real fears of cyclical history, and promised that the rule of Augustus would avert the cataclysmic destruction of the world predicted by contemporary models of historical thought.