The Philippieion, the only circular building inside the Altis, is one of the finest examples of ancient Greek architecture. Located west of the temple of Hera, it was dedicated to Zeus by Philip II of Macedon after his victory at Chaironeia in 338 BC, proving the important political role of the sanctuary at that time. After Philip's death in 336 BC, the monument was completed by his son, Alexander the Great, who had the statues of his family crafted by the famous sculptor Leochares, placed inside. The monument was also used for the worship of the deified royal family of Macedon.

The Philippieion was a particularly elegant building. Eighteen Ionic columns stood on a three-stepped marble base and supported a stone entablature. The roof had marble tiles and a bronze flower on the top. According to Pausanias, who visited the monument in the second century AD (V, 20, 9), the cella wall, built of rectangular poros blocks, was covered internally in red plaster with white joints imitating brickwork. Inside the cella were nine engaged Corinthian columns and, directly opposite the entrance, a semi-circular podium with five chryselephantine statues representing Alexander, his parents Philip and Olympias, and Philip's parents Amyntas and Euridice. The two female statues were later transferred to the Heraion, which served as a treasury, and this is where Pausanias saw them. None of these statues have survived.

Only the foundations and lower part of the walls are visible in situ. However, on the occasion of the Athens Olympic Games of 2004, the Berlin Museum returned ten of the building's architectural members (fragments of the base and columns, a Corinthian capital, part of the marble gutter with a lion's head water-spout, and a marble roof-tile) for its restoration which is currently under way.