(Isiu, Itin. Anton. p. 167; Isui, Not. Imp.), was a fort situated on the borders of the Thebaid and Heptanomis in Egypt, in lat. 27° 5′ N., and on the eastern bank of the Nile. Isium was about 20 miles SE. from the castle of Hieracon, and nearly 24 miles NE. from that of Muthis. Under the Roman empire a troop of British infantry (ala Britonum) was stationed there.