The Temple of Olympian Zeus (Greek: Ναός του Ολυμπίου Διός, Naos tou Olympiou Dios), also known as the Olympieion or Columns of the Olympian Zeus, is a colossal ruined temple in the centre of the Greek capital, Athens, that was dedicated to Zeus, king of the Olympian gods. Construction began in the 6th century BC during the rule of the Athenian tyrants, who envisaged building the greatest temple in the ancient world, but it was not completed until the reign of the Roman Emperor Hadrian in the 2nd century AD some 638 years after the project had begun.
During the Roman periods it was renowned as the largest temple in Greece and housed one of the largest cult statues in the ancient world. The temple's glory was short-lived, as it fell into disuse after being pillaged in a barbarian invasion in the 3rd century AD. It was probably never repaired and was reduced to ruins thereafter. In the centuries after the fall of the Roman Empire, it was extensively quarried for building materials to supply building projects elsewhere in the city. Despite this, a substantial part of the temple remains today, and it continues to be a major tourist attraction.
Fifteen columns remain standing today and a sixteenth column lies on the ground where it fell during a storm in 1852. Nothing remains of the cella or the great statue that it once housed. The temple was excavated in 1889-1896 by Francis Penrose of the British School in Athens (who also played a leading role in the restoration of the Parthenon), in 1922 by the German archaeologist Gabriel Welter and in the 1960s by Greek archaeologists led by Ioannes Travlos.
The temple, along with the surrounding ruins of other ancient structures, is a historical precinct administered by Ephorate of Antiquities of the Greek Interior Ministry. On 21 January 2007, a group of Hellenic neopagans held a ceremony honouring Zeus on the grounds of the temple. The event was organised by "Ellinais", an organisation which won a court battle to obtain recognition for Ancient Greek religious practices in the Autumn of 2006.