Guam (/ˈɡwɑːm/ ^ⓘ GWAHM; Chamorro: Guåhan [ˈɡʷɑhɑn]) is an island that is an organized, unincorporated territory of the United States in the Micronesia subregion of the western Pacific Ocean. Guam's capital is Hagåtña, and the most populous village is Dededo. It is the westernmost point and territory of the United States, as measured from the geographic center of the U.S. In Oceania, Guam is the largest and southernmost of the Mariana Islands and the largest island in Micronesia. In 2022, its population was 168,801. Chamorros are its largest ethnic group, but a minority on the multiethnic island. The territory spans 210 square miles (540 km²; 130,000 acres) and has a population density of 775 per square mile (299/km²).

Indigenous Guamanians are the Chamorro, who are related to the Austronesian peoples of the Malay Archipelago, the Philippines, Taiwan, and Polynesia. Unlike most of its neighbors, the Chamorro language is not classified as a Micronesian or Polynesian language. Like Palauan, it possibly constitutes an independent branch of the Malayo-Polynesian language family. The Chamorro people settled Guam and the Mariana islands approximately 3,500 years ago. Portuguese explorer Ferdinand Magellan, while in the service of Spain, was the first European to visit and claim the island in March 1521. Guam was fully colonized by Spain in 1668. Between the 17th and 19th centuries, Guam was an important stopover for Spanish Manila galleons. During the Spanish–American War, the United States captured Guam in June 1898. Under the 1898 Treaty of Paris, Spain ceded Guam to the U.S. effective April 11, 1899.

Before World War II, Guam was one of five American jurisdictions in the Pacific Ocean, along with Wake Island in Micronesia, American Samoa and Hawaii in Polynesia, and the Philippines. On December 8, 1941, hours after the attack on Pearl Harbor, Guam was captured by the Japanese, who occupied the island for two and a half years before American forces recaptured it on July 21, 1944, which is commemorated there as Liberation Day. Since the 1960s, Guam's economy has been supported primarily by tourism and the U.S. military, for which Guam is a major strategic asset. Its future political status has been a matter of significant discussion, with public opinion polls indicating a strong preference for American statehood.

Guam's de facto motto is "Where America's Day Begins", which refers to the island's proximity to the International Date Line. Guam is among the 17 non-self-governing territories listed by the United Nations, and has been a member of the Pacific Community since 1983.

Guam is called Guåhan by Chamorro speakers, from the word guaha, meaning 'to have'. Its English gloss 'we have' references the island's providing everything needed to live.

Guam and the other Mariana Islands were the first islands settled by humans in Remote Oceania. It was the first and the longest of the ocean-crossing voyages of the Austronesian peoples, and is separate from the later Polynesian settlement of the rest of Remote Oceania. They were first settled around 1500 to 1400 BC, by migrants departing from the Philippines. This was followed by a second migration from the Caroline Islands in the first millennium AD. A third migration wave took place from Island Southeast Asia, likely the Philippines or eastern Indonesia, by 900 AD.

These original settlers of Guam and the Northern Mariana Islands evolved into the Chamorro people, historically known as Chamorros after first contact with the Spaniards. The ancient Chamorro society had four classes: chamorri (chiefs), matua (upper class), achaot (middle class), and mana'chang (lower class). The matua were located in the coastal villages, which meant they had the best access to fishing grounds. The mana'chang were located in the island's interior. Matua and mana'chang rarely communicated with each other. The matua often used achaot as intermediaries.

There were also "makåhna" or "kakahna", shamans with magical powers and "'suruhånu" or "suruhåna", healers who used different kinds of plants and natural materials to make medicine. Belief in spirits of ancient Chamorros called "Taotao mo'na" still persists as a remnant of pre-European culture. It is believed that "suruhånu" or "suruhåna" are the only ones who can safely harvest plants and other natural materials from their homes or "hålomtåno" without incurring the wrath of the "Taotao mo'na." Their society was organized along matrilineal clans.