This Cabin belongs to the children of Poseidon, the god of the sea, earthquakes and horses.

Poseidon's cabin is low, long and solid, with all the windows facing the sea. The outer walls are rough gray stone with pieces of seashell and coral and look like the bottom of the ocean floor. On the inside it has six empty bunk beds and the walls glow like abalone.

Later in the series, Poseidon adds a fountain for his sons, Tyson and Percy Jackson. The fountain is made out of gray sea rock, has a fish that spouts the water from its mouth, and has a coral decoration. The bottom is filled with golden drachmas for Iris message, although it is unknown that if the coins will ever run out. Tyson later adds a few bronze Hippocampi on the ceiling as decorations. He also added unknown underwater plants and corals, sitting in the window sill, "more beautiful than the Demeter kids could whip up," Percy commented.