The Great Chicago Fire took place October 8-October 10, 1871, in Chicago, Illinois.

Despite popular legend that the Great Chicago Fire was started by a cow kicking over a lantern in the barn owned by Mrs. O'Leary on DeKoven Street, historians now believe it was begun by Daniel "Pegleg" Sullivan, who first reported the fire. Prior to his death, Sullivan confessed to starting the fire.

The Great Chicago Fire did start in Kate O'Leary's barn around 9:00 p.m. on October 8, 1871. The summer had been hot and dry, and a major fire a day earlier had left the water reserves dangerously low. When the fire was reported, neighbors hurried to protect the O'Leary's house from the blaze. High winds from the southwest caused the fire to ignite neighboring houses and move towards the center of Chicago. Between superheated winds and throwing out flaming brands, the fire crossed the Chicago River by midnight.

When the fire was extinguished two days later, the smoldering remains were too hot for a survey of the damage to be completed for a couple of days. Eventually, it was determined that the fire destroyed a patch four miles long and averaging 3/4 mile wide, more than 2,000 acres. This area included more than 28 miles of roads, 120 miles of sidewalk, 2,000 lampposts, 18,000 buildings, and $200 million in property, about a third of the city's valuation. Estimates of fatalities were in the 200-300 person range, low for such a large fire, for many had been able to escape ahead of the flames. 90,000 out of 500,000 inhabitants were left homeless.

Land speculators, such as Gurdon Saltonstall Hubbard, quickly set about rebuilding the city; and only twenty years later, Chicago hosted the World Columbian Exposition.

Several nearly simultaneous fires occurred throughout the dry Midwest that October. The Peshtigo Fire in the Green Bay area of Wisconsin began the same night and killed five times more people and burned almost the same value of property. A fire burned 80% of Holland, Michigan the same day.

See also List of famous fires.