The Sun Key is an item from Phantom Hourglass. It is found underwater in the Southwest Quadrant of the World of the Ocean King. The Treasure Chest that contains the Sun Key can be uncovered after Link has acquired the Salvage Arm from Cannon Island. The location of the Sun Key is found after Link ventures through the Temple of the Ocean King for a third time.
The Sun Key is used to open a door with a sun symbol on it in the Old Wayfarer's hideaway on Molida Island. By opening the door, Link gains access to the north end of the island where the Temple of Courage is located.