The Zayanderud or Zayandehrud (Persian: Zāyanderūd ,زاینده‌رود; from زاینده [zɑːjændɛ] "fertile" or "life-giver", and رود [rʊːd] "river"), also spelled as Zayanderud or Zayanderood, ..., is the largest river of the Iranian Plateau in central Iran.

The Zayandeh starts in the Zard-Kuh subrange of the Zagros Mountains in Chaharmahal and Bakhtiari Province. It flows 400 kilometres (249 mi) eastward before ending in the Gavkhouni swamp, a seasonal salt lake, southeast of Isfahan (Esfahan) city.

The Zayandeh used to have significant flow all year long, unlike many of Iran's rivers which are seasonal, but today it runs dry due to water extraction before reaching the city of Esfahan. In the early 2010s, the lower reaches of the river dried out completely after several years of seasonal dry-outs.

The Zayandeh River basin has an area of 41,500 square kilometres (16,000 sq mi), an altitude from 3,974 metres (13,038 ft) to 1,466 metres (4,810 ft), an average rain fall of 130 millimetres (5 in) and a monthly average temperature of 3 °C (37 °F) to 29 °C (84 °F). There are 2,700 square kilometres (1,000 sq mi) of irrigated land in the Zayandeh River basin, with water derived from the nine main hydraulic units of the Zayandeh River, wells, qanat and springs in lateral valleys. Zayandeh River water gave life to the people of central Iran mainly in Isfahan and Yazd provinces. Before the drying-out, water diverted per person was 240 litres (63 US gallons/53 imp gallons) per day in urban areas and 150 litres (40 US gallons/33 imp gallons) per day in villages. In the 1970s, the flow of the river was estimated at 1.2 cubic kilometres (0.3 cu mi) per annum, or 38 cubic metres (1,340 cu ft) per second.

People have lived on the banks of the Zayandeh for thousands of years. An ancient prehistoric culture, the Zayandeh River Culture, flourished along the banks of the Zayandeh in the 6th Millennium BC. The Zayandeh crosses the city of Isfahan, Iran. In the 17th century, Baha al-Din al-Amili, a scholar and adviser to the Safavid dynasty, designed and built a system of canals (maadi), to distribute Zayandeh water to Isfahan's suburbs. The Zayandeh riverbed is spanned by Safavid era bridges, and the river used to flow through parks.

American archaeologists and historians of Persian art, Arthur Upham Pope and his wife Phyllis Ackerman are buried in a small mausoleum on the river bank. Richard N. Frye (an American scholar of Iranian and Central Asian Studies) has also requested to be buried there.

Until the 1960s in Isfahan Province the distribution of water followed the Tomar, a document claimed to date from the 16th Century. The Tomar divided the flow of the Zayandeh River into 33 parts which were then specifically allotted to the eight major districts within the region. At the district level the water flow was divided either on a time basis, or by the use of variable weirs, so that the proportion could be maintained regardless of the height of the flow.

For centuries Isfahan city had been an oasis settlement, noted for its surrounding fertile lands and prosperity. Until the 1960s industrial demand for water was minimal, which enabled the scarce water resources to be utilized primarily for agriculture. With a growing population within the basin, and rising standards of living particularly within the city, the pressure on water resources steadily increased until the division of water Tomar was no longer feasible. The creation of large steel works and other new industries demanded water.