A cotton gin (short for cotton engine) is a machine that quickly and easily separates cotton fibers from their seeds, a job that otherwise must be performed painstakingly by hand. The fibers are processed into clothing or other cotton goods, and any undamaged seeds may be used to grow more cotton or to produce cottonseed oil.

Although simple handheld roller gins have been used since at least 500 AD, the first modern mechanical cotton gin was created by American inventor Eli Whitney in 1793, and patented in 1794. It 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. Larger, more complex, automated cotton gins remain a crucial part of the cotton industry today.

Cotton fibres are produced in the seed pods ("bolls") of the cotton plant; as a result, the fibres ("lint") in the bolls are interspersed with the seeds. The seeds must be removed from the lint in order to make the fibres useable. Historically, this task was performed by hand: production of cotton required hundreds of man-hours to separate the seeds from a useful amount of lint. Many simple seed-removing devices have been invented over the years, but most required significant operator attention and worked on a small scale.