Syr Darya (also known as Syrdarya or Sirdaryo, in Persian سیردریا) is a river in Central Asia. It rises in two headstreams in the Tien Shan mountains in Kyrgyzstan and eastern Uzbekistan and flows for some 2,220 km (1,380 miles) west and north-west through southern Kazakhstan to the remains of the Aral Sea. Along the way, it irrigates the most fertile cotton-growing region in the whole of Central Asia, together with the towns of Kokand, Khujand, Kyzyl-Orda and Turkestan . An extensive system of canals, many built in the 18th century by the Khans of Kokand, spans the regions the river flows through. Massive expansion of irrigation canals during the Soviet period, to irrigate cotton fields, wrought ecological carnage to the area, with the river drying up long before reaching the Aral Sea which, as a result, has shrunk to a small remnant of its former size. With millions of people now settled in these cotton areas (and highly repressive post-Soviet regimes in power in Kazakhstan, Uzbekistan and Turkmenistan), it is not clear how the situation can be rectified.

The name which comes from Persian and has always been used in the East is a relatively recent one in western writings; prior to the early 20th century, the river was known by its ancient Greek name, the Jaxartes. It marked the northernmost limit of Alexander of Macedon's conquests. Greek historians have claimed that here in 329 BC he founded the city Alexander Eschatê (literally, "Alexandria the Furthest") as a permanent garrison. The city is now known as Khujand.

See also Amu Darya.