Siwah (anc. Ammon or Ammonium), an oasis in N. W. Egypt, near the boundary of the disputed territory between Egypt and Tripoli, about 330 m. W. S. W. of Cairo, and about 160 m. from the coast of the Mediterranean sea; pop. about 8,000. It consists of several detached tracts, the principal of which is about 8 m. long and 3 m. broad. Its surface is undulating, rising on the north into high limestone hills. There are numerous ponds and springs, salt and fresh. The soil of the E. part is exceedingly fertile, its chief product being dates. The climate is delightful. The inhabitants are Berbers and negroes, all Mohammedans, governed by sheikhs or elders, some of whom hold office for life, others for ten years. The people understand Arabic, but have a mixed idiom of their own. Their principal town, Siwah (according to Rohlfs, who last visited it in February, 1874, in lat. 29° 12' N., lon. 25° 30' E.), is defended by a citadel on a rock, and by strong walls. The streets are irregular, narrow, and dark. It is divided into an upper and a lower town. No stranger is admitted to the former, nor are native bachelors permitted to live there. About 3 m.

S. E. of the town are the ruins of the ancient temple of Jupiter Ammon, now called Om Baydah, sculptures of Ammon, with the attributes of the ram-headed goat, being among the remains. Near the temple is what is supposed to be the fountain of the sun, a pool 80 ft. long and 55 ft. wide, formed by springs, whose water appears to be warmer by night than by day, and is heavier than that of the Nile. In the vicinity are other ruins and inscriptions of Greek, Roman, and Roman-Egyptian character. In the W. part of the oasis is a lake, called Birket Arashiah, containing an island from which strangers were till lately excluded. - In ancient times this oasis was celebrated as the seat of the oracle of Ammon. Besides the temple, with its images of Jupiter Ammon set in precious stones, it contained a royal castle surrounded by three walls, and a remarkable spring called the "fountain of the sun," the water of which was quite cold at noon and boiling hot at midnight. Cambyses made an unsuccessful attempt to take the temple. In 331 B. C. Alexander the Great marched through the desert to visit it, and the priest addressed him as the son of the god.

The emperor Justinian built here a Christian church. - See Reise zu dem Tempel des Jupiter Ammon und nach Oberagypten, by Minutoli (Berlin, 1824); "Adventures in the Libyan Desert," by Bayle Saint John (London, 1849); and Das Orakel und die Oase des Ammon, by Parthey (Berlin, 1862).