The battle against Cronus

Poseidon joined forces with Zeus and his other siblings to battle Cronus and the Titans after Zeus freed them from Cronus. In the Titanomachy, also known as the Battle of the Gods, Poseidon with his siblings, the Hecatonchires, and the Cyclopes fought against Cronus and the other Titans. The Cyclopes created Zeus' thunderbolts, and the Hecatonchires utilized rocks as weapons. Together, they also created Hades' helm of darkness and Poseidon's trident. The team represented by the younger generation of Olympians, commanded by Zeus, beat the Titans and overthrew Cronus.

Zeus, his brothers, Hades, and Poseidon made the decision to divide the universe into three sections following the Titans' War. To decide who would control which area, they drew straws. Zeus was crowned king of the skies because he pulled the longest straw. This implied that he was also in charge of both the gods and the mortals. Hades drew the shortest, so he was given the underworld, where he had control over the souls of the dead. Poseidon got the middle straw, so he was given the title "Lord of the seas" and took charge of the rivers and oceans.