The Thriasian Plain (Greek: ? , romanized: Thriasio Pedio) is a plain in western Attica, immediately to the west of Athens, in Greece. It is bounded by Mount Egaleo to the east, Mount Parnitha to the north, Mount Pateras to the west, and the Bay of Eleusina to the south.
The Thriasian Plain owes its name to the ancient deme of Thria (Greek: ?), one the demes of ancient Athens. The largest town in the plain was Eleusis (modern-day Eleusina), location of the famous Eleusinian mysteries. In Greek mythology, when the goddess Athena won the contest for control of Attica over Poseidon, Poseidon flooded the plain in wrath, until called to order by Zeus
In antiquity, as today, the area was connected to central Athens by two passes: the Sacred Way (Greek: ? ?) to the west, today used by the main Athens–Corinth highway, and another pass to the northwest nowadays used by the Attiki Odos highway. During the Peloponnesian War (431–404 BC) the Thriasian Plain was ravaged by the Spartan armies of King Archidamus II in his campaign against the Athenians.
Today it is largely an industrial area, hosting some of Greece's major industrial facilities, such as its largest oil refineries and steel mills. There are four towns in the Thriasian Plain: Eleusina (anc. Eleusis), Mandra, Magoula, and Aspropyrgos. The plain hosts a major air force base outside of Eleusina, as well as a major station of the Athens Suburban Rail. As of 2010, there are plans to create a major center for the transshipment of commercial goods unloaded in the nearby port of Pireas(Athens port) to the rest of Greece and neighboring countries.