Tanais (Greek: Τάναϊς, Tánaïs, gen.: Τανάϊδος, Tanáïdos) is the ancient name for the River Don in Russia. Strabo (Geography 11.1) regarded it as the boundary between Europe and Asia.

In antiquity, Tanais was also the name of a city in the Don river delta (Maeotian marshes) that reaches into the northeasternmost part of the Sea of Azov, which the Greeks called Lake Maeotis. The site of ancient Tanais is about 30 km west of modern Rostov on Don. The central city site lies on a plateau with a difference up to 20m in elevation in the south. It is bordered by a natural valley to the east, and an artificial ditch to the west.

The site of Tanais was occupied long before the Milesians founded an emporium there. A necropolis of over 300 burial mounds near the ancient city show that the site had already been occupied since the Bronze Age, and that mound burials continued through Greek and into even Roman times.

Greek traders seem to have been meeting nomads in the district as early as the 7th century BCE without a formal, permanent settlement. Greek colonies had two kinds of origins, apoikiai of citizens from the mother city-state, and emporia, which were strictly trading stations. Founded late in the 3rd century BCE, by merchant adventurers from Miletus, Tanais quickly developed into an emporium at the farthest northeastern extension of the Hellenic cultural sphere. It was a natural post, first for the trade of the steppes reaching away eastwards in an unbroken grass sea to the Altai, the Scythian Holy Land, second for the trade of the Black Sea, ringed with Greek-dominated ports and entrepots, and third for trade from the impenetrable north, with furs and slaves brought down the Don. Strabo mentions Tanais in his Geography (11.2.2).