A cotton gin—meaning "cotton engine"—is a machine that quickly and easily separates cotton fibers from their seeds, enabling much greater productivity than manual cotton separation. The fibers are then processed into various cotton goods such as calico, while any undamaged cotton is used largely for textiles like clothing. The separated seeds may be used to grow more cotton or to produce cottonseed oil.

Handheld roller gins had been used in the Indian subcontinent since at earliest AD 500 and then in other regions. The Indian worm-gear roller gin, invented sometime around the 16th century, has, according to Lakwete, remained virtually unchanged up to the present time. A modern mechanical cotton gin was created by American inventor Eli Whitney in 1793 and patented in 1794.

Whitney's gin used a combination of a wire screen and small wire hooks to pull the cotton through, while brushes continuously removed the loose cotton lint to prevent jams. It revolutionized the cotton industry in the United States, but also inadvertently led to the growth of slavery in the American South. Whitney's gin made cotton farming more profitable, so plantation owners expanded their plantations and used more enslaved people to pick cotton. Whitney never invented the machine to harvest cotton: it still had to be picked by hand. The invention has thus been identified as an inadvertent contributing factor to the outbreak of the American Civil War. Modern automated cotton gins use multiple powered cleaning cylinders and saws, and offer far higher productivity than their hand-powered precursors.