Upolu is an island in Samoa, formed by a massive basaltic shield volcano which rises from the seafloor of the western Pacific Ocean. The island is 75 kilometres long and 1,125 square kilometres in area, making it the second largest of the Samoan Islands by area. With approximately 145,000 inhabitants, it is by far the most populous of the Samoan Islands. Upolu is situated to the southeast of Savai'i, the "big island". Apia, the capital, is in the middle of the north coast, and Faleolo International Airport at the western end of the island. The island has not had any historically recorded eruptions, although there is evidence of three lava flows, dating back only to between a few hundred and a few thousand years ago.

Naval Base Upolu was a naval base built by the United States Navy in 1942 to support the World War II effort. The base was located on Upolu Island, Samoa in the Western Pacific Ocean, part of the Samoan Islands's Naval Base Samoa. After the surprise attack on Naval Station Pearl Harbor on December 7, 1941, the US Navy was in need of setting up more advance bases in the Pacific Ocean. At Naval Base Upolu the Navy built a sea port, an airbase and a seaplane base.