Oropos, or Oropus is a Greek seaport, on the Euripus in Attica, opposite Eretria.

In ancient times, it was a border city between Boeotia and Attica, and its possession was a continual cause of dispute between the two countries; but at last it came into the final possession of Athens, and is always alluded to under the Roman empire as an Attic town. The actual harbour, which was called Delphinium, was at the mouth of the Asopus, about a mile north of the city.

The famous oracle of Amphiaraus was situated in the territory of Oropus, 12 stadia from the city. The site has been excavated by the Greek Archaeological Society ; it contained a temple, a sacred spring, into which coins were thrown by worshippers, altars and porticoes, and a small theatre, of which the proscenium is well preserved. Worshippers used to consult the oracle of Amphiaraus by sleeping on the skin of a slaughtered ram within the sacred building.