The Provisional Government of the Territory of Jefferson was an extralegal and unrecognized United States territory that existed in the Pike's Peak mining region from October 24, 1859, until it yielded to the new Territory of Colorado on June 6, 1861. The Jefferson Territory, named for Founding Father and third United States president Thomas Jefferson, included land officially part of the Kansas Territory, the Nebraska Territory, the New Mexico Territory, the Utah Territory, and the Washington Territory, but the region was remote from the governments of those five territories.

The government of the Jefferson Territory, while democratically elected, was never legally recognized by the United States government, although it managed the territory with relatively free rein for 19 months. Many of the laws enacted by the General Assembly of the Territory of Jefferson were reenacted and given official sanction by the new Colorado General Assembly in 1861.

On August 25, 1855, the Kansas Territory created Arapahoe County, a huge county that included the entire western portion of Kansas to the Rocky Mountains. The boundaries of Arapahoe County were defined as: beginning at the northeast corner of New Mexico, running thence north to the south line of Nebraska and north line of Kansas; thence along said line to the east line of Utah Territory; thence along said line between Utah and Kansas territories, to where said line strikes New Mexico; thence along the line between said New Mexico and the territory of Kansas to the place of beginning.^{[citation needed]}

Occupied primarily by Cheyenne and Arapaho Indians with few white settlers, the county was never organized. The leaders of the Kansas Territory were preoccupied with the violent events of Bleeding Kansas, so little time or attention was available to attend to the needs of the far western portion of the territory. The question of whether to admit Kansas to the union as a slave state or free state dominated discussion in the populous eastern portion of the territory and led to three failed constitutional proposals between 1855 and 1858 (the Topeka, Lecompton and Leavenworth constitutions). The United States Congress was likewise preoccupied with threats of secession by the slave states.

In July 1858, the Pike's Peak Gold Rush began with the discovery of gold at the Dry Creek Diggings in Arapahoe County, Kansas Territory (now Englewood in Arapahoe County, Colorado). The gold rush brought 100,000 gold seekers to the area known as the Pike's Peak Country, which included Arapahoe County as well as the unorganized southwestern corner of the Nebraska Territory and parts of the New Mexico and Utah territories.

Kansas, with a growing divide between the eastern commercial centers and the central agricultural populations, had concerns over how the gold rush and the influx of miners to the Rockies could shift the base of power from the northeastern side of Kansas to the mountainous region in the west of the state. Meanwhile, the miners, being 600 miles (970 km) from the capital of the territory, felt that the legislature was out of touch with their needs. They thought a new territory or state would have the benefit of being responsive to their economic situation and consolidate the population that was currently spread across four territories. Denver area leaders decided to pursue both a relationship with Kansas and a bid for separation by sending delegates to the Kansas Territorial Legislature and the United States Congress.

On February 7, 1859, the Kansas Territorial Legislature replaced Arapahoe County with six new unorganized counties and appointed county commissioners for each. However, since the commissioners were not provided a salary, they never took office. The settlers in the region attempted to organize a county on their own and on March 28, 1859, an election was held to elect officers. A total of 774 votes were cast, including 231 from Auraria and 144 from Denver City. A desire for a new territorial government kept the elected officials from taking their offices, as doing so would have given recognition to the Kansas Territorial government. In the meantime, Hiram J Graham, the local delegate to Congress, had successfully introduced a bill to establish a new territory in Pike's Peak Country. Though the bill did not pass, it nevertheless encouraged settlers to establish a separate government themselves.

In April 1859, a small convention was held at Wootton's Hall in Auraria about the need for a local government. The name Jefferson (in honor of Thomas Jefferson, the third President of the United States who had authorized the 1803 Louisiana Purchase that included much of the proposed territory) was chosen and a constitutional convention was scheduled for June 6, 1859. The conventioneers met that day, and then adjourned until August 1, 1859, when 167 representatives from 37 districts met to draft a constitution for Jefferson State. The state constitution was subsequently rejected in a popular referendum on September 24 in favor of creating a territory, primarily because the organization of the territory would be funded by Congress while the organization of a state would be self-funded. The original authors determined to hold another convention on October 3 to draft a provisional constitution for the Territory of Jefferson.