(also Seistan; in antiquity Drangiana), a natural and historical region in Iran and Afghanistan. Sistan is a closed basin in the central part of the Iranian Plateau; the basin is composed primarily of lake deposits. In its center lie the Hamun lakes, fed by the Helmand, Khash, Farah, and Harut rivers, all of whose lower courses are in Sistan. Plains relief predominates, with an elevation of about 500 m.

Sistan has a subtropical desert climate—dry with warm winters and hot summers. Annual precipitation is less than 100 mm; maximum precipitation occurs in winter and spring. The desert vegetation, with pulvinate thornbush, provides year-round pasturage for sheep, goats, and camels. Occasional groves of tamarsk, saxaul, and Euphrates poplar are found closer to the lakes and rivers. The terminal-lake area, especially along the Helmand River, is extremely marshy; at flood stage, the rivers spill over their banks and cut new channels.

The fauna of Sistan include wild boars, jackals, and hares. Many migratory waterfowl, such as ducks, geese, herons, pelicans, and flamingos, winter on the lakes. The river valleys have oases, with orchards and fields of cotton, grains, legumes, and poppies grown for medicinal purposes. The cities of Zabol (Iran) and Zaranj (Afghanistan) are located in Sistan.

The name “Sistan” derives from the Saka, a tribe that settled in the area about 100 B.C. In the ancient and medieval periods, Sistan was held by a succession of Middle Eastern states, including the Greco-Bactrian kingdom, the Parthian empire, the Sassanid state, the Arabian Caliphate, and the Saffarid state. In the eighth and ninth centuries, Sistan was a center of the Khari-jite movement.