"The Ocean King is defender of all creation. And that temple up on the hill was built in his honor. None more worthy! Sadly, a curse was laid on the place that sucks the souls from all who enter. None dare step into the Temple of the Ocean King, so put it out of your mind!"
— Mercay Island villager

The Temple of the Ocean King is both the first and final dungeon in The Legend of Zelda: Phantom Hourglass. The temple is extremely large, having been built thirteen floors underground on Mercay Island. The temple itself has been cursed and constantly drains the life force from those bold enough to enter it. To make matters worse for those trying to enter it, the temple is full of traps and Phantoms, heavily armored foes that can down Link with a single strike of their sword, causing him to lose thirty seconds of time from the Phantom Hourglass and sending him back to the entrance of the floor he came from. Fortunately, not the entire temple is cursed, as there are areas marked with purple tiles called Safe Zones that prevent the Phantom and the life-draining effects of the temple from entering them. Throughout the dungeon, there are fallen adventurers who will give Link advice.

Link navigating the Temple of the Ocean King

After the Temple of Fire is complete, Link obtains the Phantom Hourglass on the first floor, allowing him to spend time in the temple without taking damage. However, once all the sand has drained, its protective power is lost and can only be returned after exposure to the sunlight outside the temple. Each time Link goes to the Temple of the Ocean King, with the exception of the third and the last time, he does so in search of a Sea Chart. In addition to the three types of Phantoms, there are two other enemies found in the dungeon: Phantom Eyes, which alert the Phantoms to Link's location and impede his movement, and Wizzrobes, invisible enemies that take fifteen seconds off of the Phantom Hourglass if they manage to hit him. Other varieties of Phantoms appear the further into the temple Link ventures.

Each time Link goes through the area, he must redo all of the puzzles he completed. However, because of the new items he obtains throughout his quest, it does not take as long to go through some floors, and other floors he can skip entirely. After the game is beaten, Link may participate in a time trial within the dungeon. At the end, Link is awarded with two Ship Parts. The rarity is determined by how much time is remaining. Also, each time Link reaches a certain point, he can create a warp point that can be used the next time. Upon beating Bellum, the stairway that leads to the boss chamber collapse, sealing off that area.