Balkh (Ancient Greek: Baktra or Zariaspa); was an ancient city and centre of Zoroastrianism in what is now northern Afghanistan. Today it is a small town in the province of Balkh; about 20 kilometers northwest of the provincial capital; Mazar-i Sharif; and some 74 km (46 miles) south of the Amu Darya. It was one of the major cities of Khorasan. Marco Polo described Balkh as a 'noble and great city'.

The ancient city of Balkh was under the Greeks renamed Bactra; giving its name to Bactria. It was mostly known as the centre and capital of Bactria or Takharistan. Balkh is now for the most part a mass of ruins; situated some 12 km from the right bank of the seasonally flowing Balkh River; at an elevation of about 365 m (1;200 ft).

Balkh is one of the oldest cities in the world and is considered to be the first city to which the Indo-Iranian tribes moved from the North of Amu Darya; approximately between 2000 - 1500 BC. The Arabs called it Umm Al-Belaad or Mother of Cities due to its antiquity. The city was traditionally a center of Zoroastianism. The name Zariaspa; which is either an alternate name for Balkh or a term for part of the city; may derive from the important Zoroastrian fire temple Azar-i-Asp. Balkh was regarded as the first place where Zoroaster first preached his religion; as well as the place where he died.