Sistan (sēstän´) or Seistan (sā–), border lowland region of SW Afghanistan and E Iran, c.6,000 sq mi (15,540 sq km), fed mainly by the spring flood of the Helmand River and other streams. At low water, the region is reduced to two lagoons (Hamun-i-Helmand [Sistan Lake] and Gaud-i-Zirreh [or Gowd-e Zereh]), and wheat, barley, and cotton are grown on the exposed land. Sistan's c.300,000 inhabitants live mainly on three deltas.
Sistan corresponds roughly to ancient Drangiana. In the 2d–3d cent. AD it was held by the Scythians and was called Sakastan, from which the modern name derives. From the 4th–7th cent. the region was the center of Zoroastrian worship. Sistan prospered under the Arabs from the 8th cent. AD until 1383, when Mongol conquerors destroyed the Helmand River control system and ended Sistan's prosperity. The area was disputed between Persia and Afghanistan from the 16th to early 20th cent. In times of drought, when the lake itself may dry up, water rights are still contested.