Denver is the largest city and the capital of the U.S. state of Colorado. As Denver County, it is the second most populous county in Colorado after El Paso County, which surpassed Denver County as of the 2010 Census. Denver is a consolidated city-county, located in the South Platte River Valley on the western edge of the High Plains just east of the Front Range of the Rocky Mountains. The Denver downtown district is located immediately east of the confluence of Cherry Creek with the South Platte River, approximately 12 miles (19 km) east of the foothills of the Rocky Mountains. Denver is nicknamed the Mile-High City because its official elevation is exactly one mile (1.6 km) or 5,280 feet (1,609.344 m) above sea level, which is defined by the elevation of the spot of a benchmark on the steps of the State Capital building. The elevation of the entire city ranges from 5,130 to 5,680 feet, and, some sources, such as the GNIS data and the National Elevation Dataset mark the elevation at 5,278 feet, which is reflected on various websites such as that of the National Weather Service. The 105th meridian west of Greenwich passes through Union Station and is the temporal reference for the Mountain Time Zone.

The population of Denver was 619,968 according to the 2011 Census Estimates which ranks it as the 23rd most populous U.S. city. The 10-county Denver-Aurora-Broomfield, CO Metropolitan Statistical Area had an estimated 2009 population of 2,552,195 and ranked as the 21st most populous U.S. metropolitan statistical area and the 12-county Denver-Aurora-Boulder Combined Statistical Area had an estimated 2009 population of 3,110,436 and ranked as the 16th most populous U.S. metropolitan area. Denver is the most populous city within a 500-mile (800 km) radius and the second-largest city in the Mountain West and Southwest after Phoenix. Denver is the most populous city in the Front Range Urban Corridor, an urban region stretching across 18 counties in two states. The population of the Front Range Urban Corridor was estimated to be 4,328,406 in 2009.

Denver City was founded in November 1858 as a mining town during the Pikes Peak Gold Rush in western Kansas Territory.That summer, a group of gold prospectors from Lawrence, Kansas, arrived and established Montana City on the banks of the South Platte River. This was the first settlement in what was later to become the city of Denver. The site faded quickly, however, and by the summer of 1859 it was abandoned in favor of Auraria (named after the gold mining town of Auraria, Georgia), and St. Charles City.

On November 22, 1858, General William Larimer, a land speculator from eastern Kansas, placed cottonwood logs to stake a claim on the bluff overlooking the confluence of the South Platte River and Cherry Creek, across the creek from the existing mining settlement of Auraria, and on the site of the existing townsite of St. Charles. Larimer named the town site Denver City to curry favor with Kansas Territorial Governor James W. Denver. Larimer hoped that the town's name would help make it the county seat of Arapaho County, but unknown to him Governor Denver had already resigned from office. The location was accessible to existing trails and was across the South Platte River from the site of seasonal encampments of the Cheyenne and Arapaho. The site of these first towns is now the site of Confluence Park in downtown Denver. Larimer, along with associates in the St. Charles City Land Company, sold parcels in the town to merchants and miners, with the intention of creating a major city that would cater to new emigrants. Denver City was a frontier town, with an economy based on servicing local miners with gambling, saloons, livestock and goods trading. In the early years, land parcels were often traded for grubstakes or gambled away by miners in Auraria. In May 1859, Denver City residents donated 53 lots to the Leavenworth & Pike’s Peak Express in order to secure the region’s first overland wagon route. Offering daily service for “passengers, mail, freight, and gold,” the Express reached Denver on a trail that trimmed westward travel time from twelve days to six. In 1863, Western Union furthered Denver’s dominance of the region by choosing the city for its regional terminus.

The Colorado Territory was created on February 28, 1861, Arapahoe County was formed on November 1, 1861, and Denver City was incorporated on November 7, 1861. Denver City served as the Arapahoe County Seat from 1861 until consolidation in 1902. In 1867, Denver City became the Territorial Capital. With its new-found importance, Denver City shortened its name to Denver. On August 1, 1876, Colorado was admitted to the Union.

Between 1880 and 1895 the city experienced a huge rise in corruption, as crime bosses, such as Soapy Smith, worked side by side with elected officials and the police to control elections, gambling, and the bunko gangs. The city also experienced a depression in 1893 after the crash of silver prices. In 1887, the precursor to the international charity United Way was formed in Denver by local religious leaders who raised funds and coordinated various charities to help Denver's poor. By 1890, Denver had grown to be the second-largest city west of Omaha, Nebraska, but by 1900 it had dropped to third place behind San Francisco and Los Angeles. In 1900, whites represented 96.8% of Denver's population.

In 1901, the Colorado General Assembly voted to split Arapahoe County into three parts: a new consolidated City and County of Denver, a new Adams County, and the remainder of the Arapahoe County to be renamed South Arapahoe County. A ruling by the Colorado Supreme Court, subsequent legislation, and a referendum delayed the creation of the City and County of Denver until November 15, 1902.

Denver has hosted the Democratic National Convention twice, during the years of 1908, and again in 2008, taking the opportunity to promote the city's status on the national, political, and socioeconomic stage.

Early in the 20th century, Denver, like many other cities, was home to a pioneering Brass Era car company. The Colburn Automobile Company made cars copied from the contemporary Renault.

From 1953 to 1989, the Rocky Flats Plant, a DOE nuclear weapon facility formerly located about 15 miles from Denver, produced fissile plutonium "pits" for nuclear warheads. A major fire at the facility in 1957, as well as leakage from nuclear waste stored at the site between 1958 and 1968, resulted in the contamination of some parts of Denver, to varying degrees, with plutonium-239, a harmful radioactive substance with a half-life of 24,200 years. A study by the Jefferson County health director, Dr. Carl Johnson, in 1981 linked the contamination to an increase in birth defects and cancer incidence in central Denver and nearer Rocky Flats. Later studies confirmed many of his findings. Plutonium contamination was still present outside the former plant site as of August 2010, and presents risks to building the envisioned Jefferson Parkway,^[36] which would complete Denver's automotive beltway.

Denver was selected in 1970 to host the 1976 Winter Olympics to coincide with Colorado's centennial celebration, but in November 1972 Colorado voters struck down ballot initiatives allocating public funds to pay for the high costs of the games, which were subsequently moved to Innsbruck, Austria. The notoriety of becoming the only city ever to decline to host an Olympiad after being selected has made subsequent bids difficult. The movement against hosting the games was based largely on environmental issues and was led by State Representative Richard Lamm, who was subsequently elected to three terms (1975–87) as Colorado governor. There are now talks of Denver exploring a potential bid for the 2022 Winter Olympics. In 2010, Denver adopted a comprehensive update of its zoning code. The new zoning was developed to guide development as envisioned in adopted plans such as Blueprint Denver, Transit Oriented Development Strategic Plan, Greenprint Denver, and the Strategic Transportation Plan.

Denver has also been known historically as the Queen City of the Plains and the Queen City of the West, because of its important role in the agricultural industry of the high-plains region in eastern Colorado and along the foothills of the Colorado Front Range. Several US Navy ships have been named USS Denver in honor of the city.