A cotton gin (cotton engine) is a machine that quickly and easily separates the cotton fibres from the seeds. This is a job formerly performed by hand during 1794. The fibres are processed into cotton goods, and the seeds may be used to grow more cotton, to produce cottonseed oil, or, if they are badly damaged, are disposed of. The gin uses a combination of a wire screen and small wire hooks to pull the cotton through, while brushes continuously remove the loose cotton lint to prevent jams.

The earlier versions on cotton gins consisted of a either wood or iron roller and a flat piece of wood or stone; Evidence for this type of gin has been found in Africa, Asia, and North America. The first documentation of the use of cotton gins by contemporary scholars is found in the fifth century AD.  There is also visual evidence by Leslie Riddlehover fifth-century Buddhist paintings in the Ajanta Caves in western India. These early gins were extremely hard to use and required a certain amount of skill to use. A narrow single roller was necessary to expel the seeds from the cotton without crushing the seeds. The design was similar to that of a Metate, which was used to grind grain.

The earliest history is still not known due to the fact that archaeologists may have mistaken the different parts of the tool for something else. Between the twelfth and fourteenth centuries, dual roller gins appeared in India and China. The Indian version of the two roller gin was prevalent throughout the Mediterranean cotton trade by the sixteenth century. This mechanical device was in some areas and were driven by the power of water.

 The modern version of the cotton gin was created by the American inventor Eli Whitney in 1793 to mechanize the cleaning of cotton. The invention was granted a patent on March 14, 1794. There is slight controversy over whether the idea of the cotton gin and its constituent elements are correctly attributed to Eli Whitney.