Simoeis was born to Oceanus, the Titan whose river encircles the entire world, and Tethys, his sister and wife, the goddess of the fertile waters. He was one of the three thousand rivers that Hesiod said were born from that primordial union, a brother to the Nile and the great Scamander that surrounded Troy. His stream rose from Mount Ida and ran as a rapid torrent through the plain where Troy stood. The Trojans revered him as a divine guardian of their land, for his waters nourished the fields and his daughters married into the royal house.

The defining crisis of Simoeis's mythic life came during the Trojan War, the ten-year siege of Troy by the Greeks led by Agamemnon, the king of Mycenae. When Achilles, the greatest Greek warrior, went on a rampage after the death of his beloved Patroclus, he slaughtered Trojans by the score and flung their corpses into the river Scamander. Scamander, whose divine name was Xanthos, rose up in fury, boiling with blood and dead bodies, and called upon his brother-river Simoeis for aid. He begged Simoeis to fill his streams with water from the springs, to stir up all the torrents, to raise a great wave and rouse a mighty roar of timbers and rocks, so that together they might stop the savage man who raged like a god. But before Simoeis could answer, Hera, the queen of the gods who supported the Greeks, sent Hephaestus, the lame smith-god, to set fire to the riverbanks and force Scamander back into his channel. The two rivers were subdued, and Achilles lived to fight another day.

Yet Simoeis was not merely a warrior-river. In the Iliad, Homer tells that when Hera and Athena, the grey-eyed goddess of wisdom and war, rode their chariot to the battlefield, they stopped their horses by the banks of Simoeis and let them graze on ambrosia that sprang from the river's waters. The river fed the very horses of the immortals. When the Trojans dragged the Wooden Horse into their city, the mouth of Simoeis rang aloud with the din, and when the city fell, the river rolled the shields and helmets of the dead in its current. The nymphs born of Scamander and fair-flowing Simoeis wailed over the Trojan dead, their voices rising with the waters that had nourished the city for a thousand years.

Simoeis's true legacy lay in his bloodline. His daughter Astyoche married Erichthonius, a king of the Troad, and bore Tros, the king who gave his name to Troy and the Trojans. His other daughter Hieromneme married Assaracus, another prince of Troy, and bore Capys, who fathered Anchises, the mortal lover of Aphrodite and father of Aeneas, the Trojan hero who would found the Roman race. Thus Simoeis was the great-grandfather of Aeneas, and his waters ran through the veins of the greatest dynasty in Trojan history. When Dardanos, the first ancestor of the Trojans, came to the land, he gave the firstling crop of his hair to Phrygian Simoeis, dedicating his youth to the river that would carry his descendants to empire.

In later ages, Simoeis became a poetic byword for Troy itself. Euripides used the river's name as a synonym for the whole Troad in his play Iphigenia at Aulis. Virgil remembered the river as the place where Aeneas seized a golden breastplate from the Trojan Demoleos, and where the waters rolled the armour of the brave. The river still flows today as the Dümruk Su in Turkey, emptying into the Hellespont, its name still echoing the war that made it immortal.