Hymettus, Mountain range in Attica Region, Greece

Hymettus is a mountain range in the Attica Region that extends from the eastern suburbs of Athens across several kilometers. The range features limestone formations and forested slopes that connect to various natural trails and walking paths.

Marble was quarried on Hymettus's western slopes from antiquity and used for monuments in Hellenistic and Roman times. The Kaisariani Monastery, founded in the 2nd century AD on the ruins of an ancient temple, peaked in the 12th and 13th centuries as a political and spiritual centre. The Monastery of St John the Hunter was built on the northern peak in the 12th century.

Hymettus is called Trellós ("crazy") or Trellóvouno, perhaps from the French très long said by foreign visitors about its winding length. Old Athenians were jokingly said to be mad because the sun rose from the Trellós and set at Dafni. Light songs took the mountain as their subject, such as "Up there on Hymettus there is some secret...".

Access to the summit is restricted to authorized vehicles servicing the transmitter park and military radar, so walkers stop below the ridge. Two marked loops have been cleared and signposted: route 2, 1.9 km around Korakovouni hill, and route 3, 2.5 km starting and ending at the Papagou cemetery. An asphalted road climbs toward the top.

Koutouki cave opens on the east flank of Hymettus above Paiania, not in the south; it is the largest and most notable of the caves dotted along the mountain's flanks. The smaller Liontari cave, at the north end west of Glyka Nera, takes its name from the lion of Hymettus said to have terrified the surrounding inhabitants.