Angel Island (Spanish: Isla de los Ángeles) is an island in San Francisco Bay. The entire island is included within Angel Island State Park, administered by California State Parks. The island, a California Historical Landmark, has been used by humans for a variety of purposes, including seasonal hunting and gathering by Indigenous peoples, water and timber supply for European ships, ranching by Mexicans, United States military installations, a United States Public Health Service Quarantine Station, and a U.S. Bureau of Immigration inspection and detention facility.

The Angel Island Immigration Station, on the northeast corner of the island, which has been designated a National Historic Landmark, is where officials detained, inspected, and examined approximately one million immigrants, who primarily came from Asia.

Angel Island is the second largest island in area of the San Francisco Bay (Alameda is the largest). On a clear day, Sonoma and Napa can be seen from the north side of the island; San Jose can be seen from the south side of the island. The highest point on the island, almost exactly at its center, is Mount Caroline Livermore, more commonly known as simply Mt Livermore, at a height of 788 feet (240 meters). This peak is named for Caroline Sealy Livermore. The island is almost entirely in the city of Tiburon, in Marin County, although there is a small sliver (0.7%) at the eastern end of it (Fort McDowell) which extends into the territory of the City and County of San Francisco. The island is separated from the mainland of Marin County by Raccoon Strait, the depth of the water approximately 90 feet (27 m). The United States Census Bureau reported a land area of 3.107 square miles (8.05 square kilometers) and a population of 57 people as of the 2000 census.

Angel Island emerged during the last Ice Age when the ocean, much lower and located miles to the west, shaped the landscape. The rocks of Angel Island are part of the Franciscan Complex, an extensive belt of marine sedimentary and igneous rocks which were deformed and metamorphosed during the Mesozoic Era. Metamorphism of the Franciscan Complex occurred at high pressures and low temperatures, producing indicator minerals jadeite and glaucophane, characteristic of subduction zone metamorphism. The rocks of Angel Island have been grouped with similar rocks displaying similar metamorphic minerals in the East Bay Hills and on the Tiburon Peninsula as the "Angel Island Nappe". The island's form is roughly, featuring steep ridges radiating from the central peak of the Mount Caroline Livermore. The rocks are diverse, including well-exposed serpentinite in the old quarry, sandstones and conglomerates containing clasts of glaucophane schist on Kayak Beach, meta-volcanics and cherts with dark blue amphibole and brown needles of stilpnomelane on Perles Beach. However, their relationships to one another are not well understood. The Franciscan Complex rocks are unconformably overlain by flat-lying sediments of the Colma Formation near Blunt Point on the south coast of the island. These sandstones are only weakly consolidated and are eroding to provide a supply of sand to the south coast of the island, in contrast to the northern and western beaches which are dominated by pebbles and cobbles. The shape of the hillslopes on Angel Island include the scars of prehistoric landslides and mass wasting, and deposits of eroded material may have been transported away from the island by currents in the San Francisco Bay.

Until about 10,000 years ago, Angel Island was connected to the mainland; it was cut off by the rise in sea levels due to the end of the last ice age. From about 2,000 years ago, the island was a fishing and hunting site for Coast Miwok Native Americans. Similar evidence of Native American settlement is found on the nearby mainland of the Tiburon Peninsula upon Ring Mountain. In 1775, the Spanish naval vessel San Carlos made the first European entry to the San Francisco Bay under the command of Juan de Ayala. Ayala anchored off Angel Island, and gave it its modern name (Isla de los Ángeles); the bay where he anchored is now known as Ayala Cove.

In his book Two Years Before the Mast, published in 1841, Richard Henry Dana Jr. mentions in chapter 26, that in 1834 his sailing ship collected wood from "a small island, about two leagues from the Yerba Buena anchorage, called by us 'Wood Island' and by the Mexicans 'Isla de los Ángeles' and was covered with trees to the waters edge."

It is shown, labeled I. de los Angeles, on an 1850 survey map of the San Francisco Bay Area made by Cadwalader Ringgold and an 1854 map of the area by Henry Lange. Quarry operations began in the 1850s on the east side of the island near Quarry Point, with quarried stones used in the construction of a new fortress on Alcatraz Island, a new Navy shipyard on Mare Island, and a bank in San Francisco. In 1867, General McDowell took control of the quarry and used it for Army construction at Fort Point, the San Francisco Presidio, and on Angel Island itself.

Like much of the California coast, Angel Island was subsequently used for cattle ranching. In 1863, during the American Civil War, the U.S. Army was concerned about Confederate naval raiders attacking San Francisco. It decided to construct artillery batteries on Angel Island, first at Stuart (or Stewart) Point and then Point Knox. Col. René Edward De Russy was the Chief Engineer; James Terry Gardiner was the engineer tasked with designing and supervising the work. The Army established Fort Reynolds, which was garrisoned by Battery B, 3rd Artillery Regiment. The post was named by Second Lieutenant John L. Tiernon, commander of Battery B, in honor of John F. Reynolds, a Union Army general who had been killed at the Battle of Gettysburg. This portion of the island is now known as Camp Reynolds or the West Garrison, and it subsequently became an infantry garrison during the US campaigns against Native American peoples in the West.