Amphitrite was born in the barren sea, the daughter of Nereus, the gentle Old Man of the Sea who knew all the secrets of the deep, and Doris, a daughter of Oceanus and Tethys, the primordial Titans who embodied the rivers and the sea. She was the eldest of fifty sisters, the Nereids, sea nymphs of extraordinary beauty who danced on the waves and calmed the winds for passing ships. Among them were Thetis, who would later become the mother of Achilles, the greatest warrior of the Trojan War, and Galatea, who loved the Cyclops Polyphemus. But Amphitrite stood first among them, and it was she who caught the eye of Poseidon when he saw her dancing with her sisters on the shore of Naxos, an island in the Aegean Sea.

Poseidon desired her at once, but Amphitrite was not eager to surrender her freedom. She had dedicated herself to the life of a sea-virgin, and the god of the sea had a violent temper and rough ways. She fled from him, hiding herself near Atlas, the Titan condemned to hold up the heavens, at the far edge of the Ocean stream where the world ended. Poseidon, unwilling to let her go, sent messengers to find her. Among them was Delphinus, a dolphin of such intelligence and eloquence that he could speak like a man. The dolphin tracked Amphitrite to her hiding place and persuaded her to return, speaking so well of Poseidon's power and his promise to make her queen of all the seas that she relented. She returned with Delphinus, married Poseidon, and took her place beside him in the golden palace beneath the waves. In gratitude, Poseidon placed the image of the dolphin among the stars as the constellation Delphinus, where it still shines in the northern sky.

As queen of the sea, Amphitrite bore Poseidon several children. The greatest was Triton, a god with the upper body of a man and the tail of a fish, who lived with his parents in their golden house beneath the waves and blew upon a conch shell trumpet to raise or calm the sea. She also bore Rhode, the goddess of the island of Rhodes, who married Helios, the sun god, and gave the island its name. Another daughter was Benthesicyme, whose name means Deep-Waves, and in some accounts she was also the mother of Kymopoleia, the goddess of the storm waves. But her children were not limited to gods. The ancient poets called her the mother of all sea creatures: the seals that bask on the rocks, the dolphins that race beside ships, the fish that swarm in the deep, and the shellfish that cling to the ocean floor. The poet Oppian wrote that the tribes of the sea were the "swimming brood of Amphitrite," and Homer called the seals "the brood of lovely Halosydne," one of her epithets.

Amphitrite appears in one of the most famous tests of divine parentage in Greek myth. When Theseus, the hero who had slain the Minotaur in the labyrinth of Crete, was sailing home with the Cretan princess Ariadne, the Cretan king Minos challenged his claim to be the son of Poseidon. Minos threw his golden signet ring into the sea and dared Theseus to retrieve it. Without hesitation, Theseus dived into the water. Dolphins carried him down to the palace of his father, where he found Amphitrite waiting. The poet Bacchylides describes the scene: Amphitrite put a purple cloak about the young hero and set upon his head a garland of roses that had once been a wedding gift from Aphrodite, the goddess of love. The geographer Pausanias adds that she gave him a crown bright with many gems. Theseus returned to the surface dry, wearing the ring and the crown, and Minos was forced to acknowledge his divine blood. In some accounts, Theseus later gave the crown to Ariadne as a wedding gift.

Amphitrite's name appears in one of the most sacred passages of Greek poetry. In the Homeric Hymn to Delian Apollo, she is listed among the great goddesses who stood by Leto, the mother of Apollo and Artemis, during her nine days and nine nights of labour on the island of Delos. Alongside Dione, Rhea, and Themis, Amphitrite raised a cry of joy when the god leaped forth into the light. This places her among the highest ranks of the divine, a sea-goddess honoured at the birth of one of the greatest Olympians. In later centuries her image spread across the Greek world: she rode beside Poseidon in a chariot drawn by fish-tailed horses called hippocampoi, her hair bound in a golden net, her brow adorned with the claws of a crab. The temple of Poseidon on the Corinthian isthmus contained her statue, and sailors invoked her name for safe passage across the wine-dark sea.