Arapahoe County is the third most populous of the 64 counties of the state of Colorado of the United States. The United States Census Bureau estimated that the county population was 595,546 in 2012, a 4.1% increase since 2010 census. The county seat is Littleton; the most populous city is Aurora. Arapahoe County is part of the Denver–Aurora Metropolitan Statistical Area and the Denver–Aurora–Boulder Combined Statistical Area. Arapahoe County calls itself "Colorado's First County" since its origins predate the Pike's Peak Gold Rush. The county was named for the Arapaho people (Native Americans) who once lived in the region.
On August 25, 1855, the Kansas Territorial Legislature created a huge Arapahoe County to govern the entire western portion of the Territory of Kansas. The county was named for the Arapaho Nation who lived in the region.
In July 1858, gold was discovered along the South Platte River in Arapahoe County (in present day Englewood). This discovery precipitated the Pike's Peak Gold Rush. Many residents of the mining region felt disconnected from the remote territorial governments of Kansas and Nebraska, so they voted to form their own Territory of Jefferson on October 24, 1859. The following month, the Jefferson Territorial Legislature organized 12 counties for the new territory, including a smaller Arapahoe County. Denver City served as the county seat of Arapahoe County.
The Jefferson Territory never received federal sanction, and instead on February 28, 1861, U.S. President James Buchanan signed an act organizing the Territory of Colorado. On November 1, 1861, the Colorado General Assembly organized the 17 original counties of Colorado, including a new Arapahoe County. Arapahoe County originally stretched from the line of present-day Sheridan Boulevard 160 miles (258 kilometers) east to the Kansas border, and from the line of present-day County Line Road 30 miles (48 kilometers) north to the Parallel 40° North (168th Avenue). Denver City served as the county seat of Arapahoe County until 1902.
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 reorganization until November 15, 1902. Governor James Bradley Orman designated Littleton as the temporary county seat of South Arapahoe County. On April 11, 1903, the Colorado General Assembly changed the name of South Arapahoe County back to Arapahoe County. On November 8, 1904, Arapahoe County voters chose Littleton over Englewood by a vote of 1310 to 829 to be the permanent county seat.