Tajiks are descendents of the Indo-Iranian peoples who inhabited the ancient regions of Sogdiana (in southeastern Central Asia) and Bactria (northern Afghanistan and southern Tajikistan) before recorded history. Sogdiana included the northern portion of present-day Tajikistan, and its people spoke an ancient Iranian language. In the 6th century bc Sogdiana became a satrapy of the Achaemenid Empire of ancient Persia. Alexander the Great, king of Macedonia, conquered the region in the 300s bc, but Macedonian control collapsed with Alexander’s death in 323. Sogdiana was known to the Greeks as Transoxiana. In the 100s bc Sogdiana was included in the vast empire of the Kushanas, an area that at its height stretched from Central Asia to northern India. Sogdiana was then a central hub on the Silk Road, a collective term for the ancient caravan trade routes that linked China with the Middle East, India, and imperial Rome.

The Tajiks are the direct descendants of the Iranian peoples whose continuous presence in Central Asia and northern Afghanistan is attested from the middle of the 1st millennium BC. The ancestors of the Tajiks constituted the core of the ancient population of Khwarezm (Khorezm) and Bactria, which formed part of Transoxania (Sogdiana). They were included in the empires of Persia and Alexander the Great, and they intermingled with such later invaders as the Kushans and Hepthalites in the 1st–6th centuries AD. Over the course of time, the eastern Iranian dialect that was used by the ancient Tajiks eventually gave way to Farsi, a western dialect spoken in Iran and Afghanistan.

Invasions by the Huns and the Western Turks, nomadic tribes from the north, occurred between the 4th and 6th centuries ad. Then in the 8th century, Arab invaders conquered the region and introduced Islam, which thereafter remained the predominant cultural influence. In the 9th century a peaceable and affluent Persian dynasty, the Samanids, gained control of the region. The Samanids were allied with the Sunni caliph of Baghdad, and they developed Bukhoro as an important center of Muslim culture. The Samanid dynasty weakened in the late 10th century, however, and a number of Turkic hordes, most notably the Seljuks, fought over the region until the great conquest of the Mongol emperor Genghis Khan in the 13th century. The region then became part of the vast empire of Turkic conqueror Tamerlane in the 14th century. Under Tamerlane, who established the Timurid dynasty, Samarqand (in present-day Uzbekistan) became the center of cultural and political life.