Pelusium (pĭlo͞o`shēəm), ancient city of Egypt, on the easternmost branch of the Nile (long since silted up) and c.20 mi (30 km) E of modern Port Said. It was especially important as a frontier fortress against attacks from the east. The Assyrians under Sennacherib were supposedly struck by pestilence at Pelusium, and in 525 B.C. the Persians under Cambyses overthrew Psamtik III there. There are Roman remains on the site. It is mentioned in the Bible under the name Sin.