In classical Greek mythology, the stars in the constellation of Gemini were named Castor and Pollux, known collectively as the "Dioscuri" or "Sons of Zeus." Although Leda was mother to both, the twins had different fathers. Pollux was the son of the chief Olympian God, Zeus, who had changed himself into a swan in order pursue the lovely Leda. Thus, Pollux was immortal. Castor was the son of Leda's husband, Tyndareus, King of Sparta, and thus destined to die. According to legend, they were hatched from an egg and were were brothers to the beautiful Helen of Troy (over whom the Trojan War was fought) and also brothers to Clytemnestra, Queen of Mycenae. Castor and Pollux were known to be well-educated, strong and daring, while remaining gentle individuals. They were celebrated as healers, physicians and protectors of humankind. Castor and Pollux were also Argonauts, being among the crew who sailed with Jason in the Quest for the Golden Fleece. During that voyage, a fierce storm threatened the mission but abated as a pair of stars appeared over the heads of the twins. Since that time, seamen have called upon the brothers for protection from peril and the eerie lightning phenomenon sometimes called Saint Elmo's Fire has long been regarded as the spirits of the twins playing in the sails. It is considered by sailors to be a most favorable omen. Castor and Pollux were also counted among the number who fought at the Siege of Troy.

Castor (a horseman) and Pollux (a boxer) fell in love with beautiful sisters, daughters of Leudippus, who were already betrothed to suitors. According to the myth, these suitors, named Idas and Lynceus, were reputed cousins of Castor and Pollux by virtue of Poseidon, God of the Sea, who was rumored to be their father. The twins challenged Idas and Lynceus in battle and slew their rivals, but Castor was mortally wounded by Idas in the process. Overcome with grief, Pollux would have committed suicide in order to be with his beloved brother, but since he was an immortal, this proved to be impossible. Finally, Zeus placed both their immortal souls together in the sky as symbols of brotherly love...the precept they had demonstrated so gallantly throughout their lives on Earth. However, this gift did not come without a price. Zeus determined that for the rest of eternity, the twins would have to alternate between the light and the dark...spending one day on Olympus and then the next day in Hades.

Many statues and temples were once erected to Castor and Pollux and they were often carved as figureheads on ships. The Apostle Paul sailed on one such adorned ship from the Isle of Melita during his journey to Rome. Images representing this constellation have been found on both Greek and Roman coins, and the image of the twins also appeared on Babylonian boundary stones as well as charts of the heavens. Known to be handsome and adventurous young men, they perpetuate the memories of a roving and carefree life by acting as the protectors of sailors and of hospitality. Their nature was said to be energetic and volatile, vain, talktative and curious. In modern times, the twins have once again been ingrained in history as a critical part of Earth's journey into space when Project Gemini placed into orbit two men aboard one spacecraft.