Altar of Zeus Agoraios, Religious monument in Ancient Agora, Greece.

The Altar of Zeus Agoraios is a white marble altar, 9 m deep and 5.5 m wide, in the northwest of the Ancient Agora. Its foundations are of solid cobblestones, the outer structure of Piraeus poros, and the four base courses of Pentelic marble. A 2.90 m stone upright survives, and fragments of a matching upright were found in a well at the Metroon propylon.

Built in the 4th century BC, the altar first stood on the Pnyx, the hill where Athenian assemblies met. Mason's marks from the Augustan period show it was later moved to the Agora. Excavations in 1931, one of the first discoveries in the Agora, uncovered its remains. T. L. Shear Jr. linked the move to Augustus curtailing the powers of the Athenian assemblies.

The epithet Agoraios names Zeus as protector of the assembly place, so the altar stood where citizens gathered to deliberate. Its move from the Pnyx to the Agora under Augustus, as T. L. Shear Jr. noted, came just as the emperor curtailed the powers of those assemblies.

The altar sits within an archaeological site that is easy to reach on foot among other ancient ruins and remains. Visitors should protect themselves from sun and weather and wear comfortable shoes since the ground is uneven throughout the area.

The altar originally stood at the Pnyx, a place of political meetings, and was later moved to the Agora, showing how its role was reinterpreted. This relocation and reuse at a different religious and commercial center makes it a rare example of how ancient structures were adapted for new purposes.