The Philippine Sea is a marginal sea of the Western Pacific Ocean east of the Philippine Archipelago and the largest sea in the world, occupying an estimated surface area of 5 million square kilometers (2×10^⁶ sq mi). The Philippine Sea Plate forms the floor of the sea. Its western border is the first island chain to the west, comprising the Ryukyu Islands in the northwest and Taiwan in the west. Its southwestern border comprises the Philippine islands of Luzon, Catanduanes, Samar, Leyte, and Mindanao. Its northern border comprises the Japanese islands of Honshu, Shikoku and Kyūshū. Its eastern border is the second island chain to the east, comprising the Bonin Islands and Iwo Jima in the northeast, the Mariana Islands (including Guam, Saipan, and Tinian) in the due east, and Halmahera, Palau, Yap and Ulithi (of the Caroline Islands) in the southeast. Its southern border is Indonesia's Morotai Island.

The sea has a complex and diverse undersea relief. The floor is formed into a structural basin by a series of geologic faults and fracture zones. Island arcs, which are actually extended ridges protruding above the ocean surface due to plate tectonic activity in the area, enclose the Philippine Sea to the north, east and south. The Philippine archipelago, Ryukyu Islands, and the Marianas are examples. Another prominent feature of the Philippine Sea is the presence of deep sea trenches, among them the Philippine Trench and the Mariana Trench, containing the deepest point on the planet.

The Philippine Sea has the Philippines and Taiwan to the west, Japan to the north, the Marianas to the east and Palau to the south. Adjacent seas include the Celebes Sea which is separated by Mindanao and smaller islands to the south, the South China Sea which is separated by Philippines, and the East China Sea which is separated by the Ryukyu Islands.

The International Hydrographic Organization defines the Philippine Sea as "that area of the North Pacific Ocean off the Eastern coasts of the Philippine Islands", bounded as follows:^{[failed verification]}

On the west. By the eastern limits of the East Indian Archipelago, South China Sea and East China Sea.

On the north. By the southeast coast of Kyushu, the southern and eastern limits of the Inland Sea and the south coast of Honshu Island.

On the east. By the ridge joining Japan to the Bonin, Volcano and Ladrone (Mariana) Islands, all these being included in the Philippine Sea.

On the south. By a line joining Guam, Yap, Pelew (Palau) and Halmahera Islands.