The Megarian Decree was a set of economic sanctions levied upon Megara c. 432 BC by the Athenian Empire shortly before the outbreak of the Peloponnesian War. This move is considered one of the first uses of economics as a foreign policy tool. The decree addressed the Megarians' supposed trespass on land sacred to Demeter known as the Hiera Orgas, the killing of the Athenian herald who was sent to their city to reproach them, and giving shelter to slaves who had fled from Athens. The Megarian decree effectively blocked Megara from trading in any port within the Delian League, isolating the city and greatly damaging its economy. The exact influence the Megarian Decree had on the beginning of the Peloponnesian War is a matter that is highly debated to this day.

Megara is a historic town and a municipality in West Attica, Greece. It lies in the northern section of the Isthmus of Corinth opposite the island of Salamis, which belonged to Megara in archaic times, before being taken by Athens. Megara was one of the four districts of Attica, embodied in the four mythic sons of King Pandion II, of whom Nisos was the ruler of Megara. Megara was also a trade port, its people using their ships and wealth as a way to gain leverage on armies of neighboring poleis. Megara specialized in the exportation of wool and other animal products including livestock such as horses. It possessed two harbors, Pagae to the west on the Corinthian Gulf, and Nisaea to the east on the Saronic Gulf of the Aegean Sea.