The Dayuan or Ta-Yuan (Chinese: 大宛; Pinyin: Dàyuān; Wade-Giles: Ta-Yuan, lit. “Great Yuan”) were a people of Ferghana in Central Asia, described in the Chinese historical works of Records of the Grand Historian and the Book of Han, which follow the travels of Chinese explorer Zhang Qian in 130 BCE and the numerous embassies that followed him into Central Asia thereafter. The country of Dayuan is generally accepted as relating to the Ferghana Valley.
These Chinese accounts describe the Dayuan as urbanized dwellers with Caucasian features, living in walled cities and having "customs identical to those of the Greco-Bactrians", a Hellenistic kingdom that was ruling Bactria at that time in today’s northern Afghanistan. The Dayuan are also described as manufacturers and great lovers of wine.
The Dayuan were probably the descendants of the Greek colonies that were established by Alexander the Great in Ferghana in 329 BCE, and prospered within the Hellenistic realm of the Seleucids and Greco-Bactrians, until they were isolated by the migrations of the Yueh-Chih around 160 BCE. It has also been suggested that the name “Yuan” was simply a transliteration of the words “Yona”, or “Yavana”, used throughout antiquity in Asia to designate Greeks (“Ionians”), so that Dayuan (lit. “Great Yuan”) would mean "Great Ionians".
The interaction between the Dayuan and the Chinese is historically crucial, since it represents one of the first major contacts between an urbanized Indo-Iranian culture and the Chinese civilization, opening the way to the formation of the Silk Road that was to link the East and the West in material and cultural exchange from the 1st century BCE to the 15th century.