The Syr Darya^{[pronunciation?]} (Persian: سيردريا‎; Tajik: Сирдарё; Kazakh: Сырдария ; Arabic: سيحون‎; Uzbek: Sirdaryo), also transliterated Syrdarya or Sirdaryo, is a river in Central Asia, sometimes known as the Jaxartes or Yaxartes^{[pronunciation?]} from its Ancient Greek name Ἰαξάρτης. The Greek name is derived from Old Persian, Yakhsha Arta ("Great Pearly"), a reference to the color of the river's water. In medieval Islamic writings, the river is uniformly known as Sayhoun (سيحون) - after one of the four rivers of Paradise. (Amu Darya was likewise known as Jayhoun, the name of another one of the four).

The name, which comes from Persian and has long been used in the East, is a relatively recent one in western writings; prior to the early 20th century, the river was known by various versions of its ancient Greek name. Following the Battle of Jaxartes the river marked the northernmost limit of Alexander the Great's conquests. Greek historians have claimed that here in 329 BC he founded the city Alexandria Eschate (literally, "Alexandria the Furthest") as a permanent garrison. The city is now known as Khujand. In reality, he had just renamed (and possibly, expanded) the city of Cyropolis founded by king Cyrus the Great of Persia, more than two centuries earlier.

Syr may refer to: