Idaho  is a state in the northwestern region of the United States. Idaho is the 14th largest, the 39th most populous, and the 7th least densely populated of the 50 United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state.

Idaho is a mountainous state with an area larger than that of all of New England. It is surrounded by the states of Washington, Oregon, Nevada, Utah, Wyoming, Montana, and the Canadian province of British Columbia. The network of dams and locks on the Columbia River and Snake River make the city of Lewiston the farthest inland seaport on the Pacific coast of the contiguous United States.

Idaho's nickname is the "Gem State", because nearly every known type of gemstone has been found there. In addition, Idaho is one of only two places in the world where star garnets can be found in any significant quantities, the other being India. Idaho is sometimes called the "Potato State" owing to its popular and widely distributed crop. The state motto is Esto Perpetua (Latin for "Let it be forever").

Idaho borders six states and one Canadian province. The states of Washington and Oregon are to the west, Nevada and Utah are to the south, and Montana and Wyoming are to the east. Idaho also shares a short border with the Canadian province of British Columbia to the north. The landscape is rugged with some of the largest unspoiled natural areas in the United States. For example, at 2.3 million acres (930,000 ha), the Frank Church-River of No Return Wilderness Area is the largest contiguous area of protected wilderness in the continental United States. Idaho is a Rocky Mountain state with abundant natural resources and scenic areas. The state has snow-capped mountain ranges, rapids, vast lakes and steep canyons. The waters of the Snake River rush through Hells Canyon, the deepest gorge in the United States. Shoshone Falls plunges down rugged cliffs from a height greater than that of Niagara Falls. The major rivers in Idaho are the Snake River, the Clark Fork/Pend Oreille River, the Clearwater River, the Salmon River. Other significant rivers include the Coeur d'Alene River, the Spokane River, the Boise River, and the Payette River. The Salmon River empties into the Snake in Hells Canyon and forms the southern boundary of Nez Perce County on its north shore, of which Lewiston is the county seat. The Port of Lewiston, at the confluence of the Clearwater and the Snake Rivers is the farthest inland seaport on the West Coast at 465 river miles from the Pacific at Astoria, Oregon.

Idaho's highest point is Borah Peak, 12,662 ft (3,859 m), in the Lost River Range north of Mackay. Idaho's lowest point, 710 ft (216 m), is in Lewiston, where the Clearwater River joins the Snake River and continues into Washington. The Sawtooth Range is often considered Idaho's most famous mountain range. Other mountain ranges in Idaho include the Bitterroot Range, the White Cloud Mountains, the Lost River Range, the Clearwater Mountains, and the Salmon River Mountains.

Idaho has two time zones, with the dividing line approximately midway between Canada and Nevada. Southern Idaho, including the Boise metropolitan area, Idaho Falls, Pocatello, and Twin Falls, are in the Mountain Time Zone. A legislative error (15 U.S.C.ch. 6 §264) theoretically placed this region in the Central Time Zone, but this was corrected with a 2007 amendment. Areas north of the Salmon River, including Coeur d'Alene, Moscow, Lewiston, and Sandpoint, are in the Pacific Time Zone, which contains less than a quarter of the state's population and land area.