Gemini is one of only two signs whose myth has some pictorial connection with the pattern of stars that make up its constellation. It was so called because of the two bright stars it contains. The Babylonians called it the Great Twins in their Zodiac. The myth most strongly associated with the Gemini sign is that of Castor and Pollux, known as the Dioscuri, which means "the young sons of Zeus." The paternity of Castor and Pollux is in fact a complicated affair.

While their mortal parents were Leda and Tyndareus a union between Zeus (Jupiter), the king of the gods, while disguised as a swan and Leda confuses the issue. Leda produced two eggs with twins in each. From one egg came Pollux and Helen; from the other egg came Castor and Clytemnestra. Pollux and Helen (later to be known as Helen of Troy) were said to be the children of Zeus and therefore immortal, whereas Castor and Clytemnestra were assumed to be the mortal children of Tyndareus. It was in fact quite common for people of ancient civilizations to claim that one child from a pair of twins were of divine origin.

Castor and Pollux were brought up in Sparta, where they formed a very close friendship. Among their exploits together, Castor and Pollux rescued their sister Helen from an abductor, Theseus and joined Jason and the Argonauts expedition to recover the Golden Fleece. Afterward the two boys fell in love with the two sisters Hilaeira and Phoebe who were already betrothed at the time. When the boys carried them off, Castor was tragically killed in the ensuing brawl. Pollux, the immortal brother could not bear the thought of being parted from his mortal brother and wept over his body.

Touched by the sight of such brotherly devotion, Zeus allowed Pollux to share his immortality with Castor. As a result, the twins spent half their time in the Underworld with the spirits of the dead and half with the gods on Mount Olympus. Great athletes when on Earth, Castor and Pollux became patrons of all athletic contests. They were also said to protect sailors from the rough sea and to whom they still appear during storms as the lights of St. Elmo's fire. This belief dates back to the occasion when Zeus saved the Argonauts from a violent storm that was threatening to sink their ship, the Argo.

Two flames came down from the heavens and hung above the heads of the Dioscuri signaling the end of the storm. In later times, Castor and Pollux were regarded as divine and according to legend were supposed to ride through the sky on two white horses, carrying dazzling spears, each with a star above his brow. During the years of Imperial Rome, Castor and Pollux were believed to descend to Earth in order to fight at the head of the Roman army whenever it did battle with its enemies.