The river Eridanos (or Eridanus) is an imagined river of Greek mythology whose name has been adopted by paleogeographers to describe the real ice age river that ran in the bed of the Baltic Sea.

Mythic Eridanos is mentioned in ancient Greek writings as a river in northern Europe rich in amber.

Hesiod, in Theogony, calls it " deep-eddying Eridanos" in his list of rivers, the offspring of Tethys. In Dionysiaca, the vast monster Typhon boasts that he will bathe in "starry Eridanus." Herodotus (III, 115) points out that the word Eridanos is essentially Greek in character, and surmises that consequently the river supposed to run round the world is probably a Greek invention. He associated it with the river Po, because the Po was located near the end of the Amber Trail. Amber originated from the tears of the Heliades shed when their brother, Phaeton, died and fell from the sky and tumbled into the Eridanos (Ovid, Metamorphoses II, 367-380), where, “along the green banks of the river Eridanos,” King Cygnus mourned him and was transformed into a swan. There in the far west, Heracles asked the river nymphs of Eridanos to help him locate the Garden of the Hesperides.

Strabo disregards such mythmaking:

"One must put aside many of the mythical or false accounts such as those of Phaethon and of the Heliades changed into black poplars near the Eridanos (a river that does not exist anywhere on earth, although it is said to be near the Po), and of the Islands of Amber that lie off the Po, and of the guinea-fowl on them, because none of these exist in this area." —Geography V,1,9.

Eridanus ("the river") was considered one of the rivers of Hades by Virgil in his Aeneid VI, 659.

Athenian Eridanos. Eridanos is also the small stream that flowed from a source in the foothills of the Lykabettos, through the Agora of ancient Athens, to the archaeological site of the Kerameikos, where its bed is still visible. Its course had been for the most part covered since ancient times, and was only visible outside the ancient walls in the district of Kerameikos [1]. In the 1990s it was rediscovered during the excavations for the Athens Metro subway, and its waters caused considerable technical problems at times.

• Eridanos, the river of ancient Athens: archaeological guide (in Greek and English) (Archaeological Receipts Fund). ISBN 960-214-293-6

Geologists' Eridanos. The name was given by geologists to a river which flowed in what is now the bed of the Baltic Sea. The geological Eridanos was most important about one million years ago, when it had a length of about 2,700 kilometres. It began in Laponia, and then flowed through the area of the modern-day Gulf of Bothnia to western Europe, where it had an immense delta which spanned almost the entire current North Sea. It was comparable in size to the current-day Amazon River.

The Eridanos began about 40 million years ago. About 12 million years ago the river reached the North Sea area, where it began to build an immense delta with its sediments. The Eridanos disappeared during the first Ice age of 700,000 years ago, which completely covered the riverbed. By the time the ice caps retreated the ancient river valley had been widened into the current-day Baltic Sea.

Remnants of the Eridanos are found all through northern Europe, from the Netherlands at its western end to sediments in northern Lapponia.