The Union Colony of Colorado (also called the Greeley Colony and The Union Temperance Colony) was a 19th-century private enterprise formed to promote agricultural settlements in the South Platte River valley in the Colorado Territory. Organization of the colony began in October 1869 by Nathan Meeker in order to establish a religiously-oriented utopian community of "high moral standards". The colony was founded in March 1870 at the site of present-day Greeley, Colorado. Union Colony was financially backed and promoted by New York Tribune editor, Horace Greeley, a prominent advocate of the settlement of the American West. The homesteaded colony greatly advanced irrigation usage in present-day northern Colorado, demonstrating the viability of cultivation at a time when agriculture was emerging as a rival to mining as the principal basis for the territorial economy.
Horace Greeley had journeyed west in 1859 at the height of the Colorado Gold Rush, going from Denver to Fort Laramie via the Overland Stage Line before continuing on to California. Greeley's route took him up the valleys of the South Platte and Cache la Poudre near the eventual location of the colony.