10 Largest US Islands

No two islands are identical. Most islands were historically connected to the mainland, resulting in their environmental characteristics often reflecting those of their respective regions. For example, the islands of Alaska exhibit a colder, humid climate consistent with the state's overall conditions, whereas the Hawaiian Islands are characterized by a combination of tropical and arid climates. However, the inherently smaller size of islands and their complete encirclement by water impart distinctive traits. Furthermore, an island's geographical location significantly influences its physical features, human activities, and environmental dynamics.

The largest United States Island is Hawaii Island at 4,028 mi². Following behind it are Kodiak Island, Puerto Rico, and Prince of Wales Island at 3588 mi², 3513 mi², and 2577 mi², respectively. Massive geologic features that are common to these ten islands include mountains with elevations over 1,200 miles, as well as dramatic fjords and bays that cut into the landscape itself.

The 10 Largest US Islands Ranked By Area

1- Hawaii Island - 4,028 mi²

This accurately named "Big Island" of the Hawaiian chain formed a little less than 500,000 years ago, making it the youngest in the group. A hotspot, a fixed plume of mantle material, formed the islands as the Pacific Plate glided over it, like a stationary piping bag dropping frosting on a rotating cake. The volcano that formed Hawaii Island, Mauna Loa, is the largest subaerial volcano in mass and volume on Earth; subaerial simply means a volcano that erupts above the Earth's surface, on land, rather than underwater or beneath ice.

2- Kodiak Island - 3,588 mi²

Kodiak Island, the second-largest island in the United States, lies off the southern coast of Alaska. Like the majority of islands on this list, it was shaped by tectonic uplift and glacial activity over millions of years. Peaks exceeding 4,000 feet tower above dense forests in the north and east, while the southern regions remain barren and windswept. Kodiak’s coastline is made up of deep fjords and inlets carved by retreating glaciers, supporting one of the richest marine ecosystems in the world. Despite its size, human settlement is sparse, concentrated primarily in the city of Kodiak, with a population of around 6,000.

3- Puerto Rico - 3,513 mi²

Even though Puerto Rico is the smallest and easternmost island of the Greater Antilles, it still spans 3,513 mi² making it the third-largest US island. Formed millions of years ago at the boundary of the Caribbean and North American tectonic plates, it features a central mountain range, the Cordillera Central, with peaks exceeding 4,300 feet. Its topography includes karst landscapes and over 300 beaches along its extensive coastline. Unlike the geologically younger Hawaiian islands, Puerto Rico’s formation is linked to plate convergence, not a volcanic hotspot. Last, Puerto Rico's size supports over 3 million residents, and its central Caribbean location was extremely useful to Spain before it became a US territory in 1898.

4- Prince of Wales Island - 2,577 mi²

The mountainous landmass of Prince of Wales Island sits north of the Hecate Strait in southeastern Alaska. It is heavily wooded on the surface, and geologically, it has extensive limestone formations and caves like the El Capitan Pit, one of the deepest vertical shafts in the United States. Steep-sided mountains, often exceeding 3,800 feet, dominate the island, while dense temperate rainforests blanket much of the landscape. Its deeply indented coastline features countless inlets and bays, and these harbor valuable populations of marine life. Conveniently, the sparse human population of the island has worked in favor of preserving the island's natural ecosystems.

5- Chichagof Island - 2,080 mi²

Chichagof Island is part of Alaska's Alexander Archipelago, and its surface is marked by sharp elevations and numerous water channels, creating a network of fjords and bays that slice into its landmass. The island is situated near the eastern edge of the Pacific Ring of Fire, and because it has 741 miles of shoreline, it is perpetually threatened by tsunamis and other seismic activities in the region. For example, in 1927, an earthquake of magnitude 7.1 hit the northern section of Chichagof island and caused disruptions in the environment and for the Hoonah population, which was likely around a thousand persons at the time.

6- St. Lawrence Island - 1,983 mi²

The Bering Sea, 36 miles east of Russia, is home to St. Lawrence Island. Believed to be a remnant of the Bering Land Bridge from the Pleistocene epoch, the island may have once been traversed by the original populations who first inhabited North and South America. The island's terrain includes tundra and volcanic features, with its highest elevation reaching 2,066 feet at Atuk Mountain. Interestingly, the island is still home to the Siberian Yupik people, who have sporadically settled there in the last 2,000 years.

7- Admiralty Island - 1,684 mi²

Alaska's Alexander Archipelago is home to another mention on this list, Admiralty Island. The island stretches about 90 miles in length and 34 miles in width, and it possesses elevations measuring up to 4,650 miles. The island is nearly bisected by the 39-mile-long Seymour Canal, with the extremely remote Glass Peninsula extending to the east. Around 90% of Admiralty Island is protected by the Kootznoowoo Wilderness Preservation, which allows old-growth forests to thrive. Better yet, the island is famous for remaining home to bald eagles and a massive population of 1,600 brown bears, giving it one of the highest concentrations relative to anywhere else in the world.

8- Nunivak Island - 1,625 mi²

Situated about 30 miles offshore from the Yukon River delta in Alaska, Nunivak Island is formed out of a volcanic plateau rising over 524 feet above sea level. The island hosts around 60 cinder cones and four maars, with widespread thin pahoehoe lava flows originating from small shield volcanoes. These geological points of interest are evidence of volcanic activity that began approximately 6.1 million years ago. A herd of 4,000 reindeer, first introduced in 1920, roams the island alongside indigenous inhabitants. There are also a number of great shaggy musk oxen that originate from Greenland after the Alaskan musk ox became extinct in the 1800s.

9- Unimak Island - 1,590 mi²

Unimak Island, the largest in Alaska's Aleutian chain, spans 1,590 square miles. It features Mount Shishaldin, a highly active volcano rising to 9,373 feet, noted for its near-perfect conical shape. The island also contains Fisher Caldera, a significant volcanic crater measuring about 6.8 by 11.1 miles, formed by a massive eruption around 9,100 years ago. Unimak's geological activity is part of the Aleutian Arc, where the Pacific Plate subducts beneath the North American Plate, leading to frequent seismic events. Population-wise, the last census recorded roughly 35 people occupying the island in 2010.

10- Baranof Island - 1,570 mi²

Baranof Island's terrain includes Mount Bassie, rising to 4,301 feet, and Mount Furuhelm at 3,622 feet. There are also numerous lakes on the island, such as Baranof Lake and Blue Lake, and rivers like the Baranof and Indian Rivers. Historically, the island was named by an Imperial Russian Navy captain in honor of Alexander Andreyevich Baranov, who was the first governor of Russian America between 1799 and 1818. In 1867, the island was purchased by America from Russia as part of the Alaska Purchase. In 1910, Baranof Island became a national monument, and then in 1972, it was designated as a national historical park.

Each US Island territory is a story of the country's historical growth and unique politics, the memories of the Indigenous persons who often still inhabit those lands, and the cataclysmic geological forces that first formed each island. Whether by hotspot or tectonic plate, each island is part of larger natural movements that continue to either create new islands or shape these islands. In the distant future, on the scale of millions of years, these islands may change significantly in size and appearance, and some may vanish into the ocean altogether.

The Largest Islands In The United States Of America (over 75 square miles)