Seeadler Harbor, also known as Port Seeadler, is located on Manus Island, Admiralty Islands, Papua New Guinea and played an important role in World War II. In German, "Seeadler" means sea eagle, pointing to German colonial activity between 1884 and 1919 in that area. The bay was named in 1900 after the German cruiser SMS Seeadler.

Manus Island is part of Manus Province in northern Papua New Guinea and is the largest of the Admiralty Islands. It is the fifth-largest island in Papua New Guinea, with an area of 2,100 km2 (810 sq mi), measuring around 100 km × 30 km. Manus Island is covered in rugged jungles which can be broadly described as lowland tropical rain forest. The highest point on Manus Island is Mt. Dremsel, 718 metres (2,356 ft) above sea level at the centre of the south coast. Manus Island is volcanic in origin and probably broke through the ocean's surface in the late Miocene, 8 to 10 million years ago. The substrate of the island is either directly volcanic or from uplifted coral limestone.