Seistan (anc. Sacastane, the country of the Sacoe), a province in the S. W. part of Afghanistan, with an adjoining part included in Persia. It is between lat. 30° and 32° N., and lon. 61° and 63° E., in the lower basin of the river Helmund. The lake of Seistan, or Ha-moon (in its S. W. part known as Lake Zurrah or Zirreh), into which flow the Helmund from the south, the Khash-rud from the east, and the Furrah-rud and the Harut from the north, is a low and swampy expanse from 15 to 30 m. wide, and nearly 200 m. long from N. to S., mainly between the 61st and 62d meridians. A great part of this depressed area, partly included in Persia and Beloochistan, is now dry, though subject to inundation. In the north, near lat. 31° 30', it is occupied by two shallow and reedy lakes, about 15 m. apart, each of which is also called Hamoon. Seistan proper is a well watered and fertile alluvial plain of sand and clay W. of the Helmund in the lower part of its northward course, bounded S. by the main irrigation canal, and N. and W. by the Hamoon; estimated area, 947 sq. m.; pop. about 45,000, of whom 20,000 are Seistanis, the purest type of Aryan Persians, and 10,000 nomadic Beloochees. The Afghans are few, but politically powerful. "Wheat, barley, and melons are produced abundantly, with some cotton, peas, beans, and oil-seeds. Snow rarely falls, but the winters are windy and the mercury sinks to 5° F., rising above 90° in spring and summer.

Traces of an elaborate civilization abound, and among the numerous ruins those of Zaranj, the ancient capital as the Arab writers call it, are the most celebrated. The principal existing towns are in the district watered by the main canal; among them is Sekuha, the modern capital. Outer Seistan is 30 m. wide, and extends from the mouth of the Helmund about 120 m. S. along the right or E. bank. It also includes a plain about 80 m. long and 40 m. broad, stretching southward from Seistan . proper. The only important town is Charkansur, S. of the Khash-rud, containing a fort and 150 houses. - Seistan was in antiquity a part of Drangiana or Zarangia. It is believed that the Aria Palus of Ptolemy was the lake of Seistan. Some time before the Christian era it was overrun by Scythian hordes, of which the paramount tribe were the Saca3, who gave their name to the country. The Scythians were overcome at the time of the Arab invasion, and Seistan afterward became a province of Persia. It now belongs mainly to Afghanistan. In 1871 an arbitration commission under Sir F. J. Goldsmid fixed the boundary so as to give Persia nearly all of Seistan proper.