Kodachrome is the brand name for a color reversal film introduced by Eastman Kodak in 1935. It was one of the first successful color materials and was used for both cinematography and still photography. For many years, Kodachrome was widely used for professional color photography, especially for images intended for publication in print media.

Because of its complex processing requirements, the film was initially sold only with the cost of processing; independent photography stores were prohibited from developing Kodachrome. To develop the film, customers had to mail it to Kodak, which would then send the developed film back as part of the purchase price. In 1954, the U.S. Department of Justice found that this practice violated antitrust laws by being uncompetitive. Kodak then entered into a consent decree, requiring the company to offer Kodachrome film for sale without the development fee, as well as license Kodachrome development patents to independent photography stores. Kodak had sold mailers to users who wanted their films to be processed by them. Nonetheless, the process-paid arrangement continued in other markets around the world.

Eventually, the growth and popularity of alternative photographic materials, and, much later, the widespread transition to digital photography, led to Kodachrome's loss of market share. Its manufacture was discontinued in 2009, and processing ended in December 2010. In early 2017, Kodak announced it was investigating the possibility of re-introducing Kodachrome, but later conceded that this was unlikely to happen.

Kodachrome was the first color film to be successfully mass-marketed that used a subtractive color method. Previous materials, such as Autochrome and Dufaycolor, had used the additive screenplate methods. Until its discontinuation, Kodachrome was the oldest surviving brand of color film. It was manufactured for 74 years in various formats to suit still and motion picture cameras, including 8mm, Super 8, 16mm for movies (exclusively through Eastman Kodak), 35mm for movies (exclusively available through Technicolor Corp as "Technicolor Monopack") and 135, 120, 110, 126, 828 and large format for still photography.

Kodachrome is appreciated in the archival and professional market for its accurate color reproduction and dark-storage longevity. Because of these qualities, it was used by Walton Sound and Film Services in the UK in 1953 for the official 16mm film of the coronation of Elizabeth II of the United Kingdom. Copies of the film for sale to the public were also produced using Kodachrome. More recent professional film photographers such as Steve McCurry, David Alan Harvey, Peter Guttman and Alex Webb also favored the film. It was used by McCurry for his iconic 1984 portrait of Sharbat Gula, Afghan Girl, for National Geographic magazine.

Before Kodachrome film was marketed in 1935 most color photography had been achieved using films that employed additive color methods and materials, such as Autochrome and Dufaycolor, These first practical color processes had several disadvantages because they used a Réseau filter made from discrete color elements that were visible upon enlargement. The finished transparencies absorbed between 70% and 80% of light upon projection, requiring very bright projection lamps, especially for large projections. Subtractive color methods avoided these disadvantages.

Rudolf Fischer was granted a patent in 1913 for a proposed color photography process in which three separate emulsions, each sensitive to a different color, would be exposed simultaneously. However, Fischer was unable to implement the idea.

The first Kodak product called Kodachrome was invented by John Capstaff in 1913. His Kodachrome was a subtractive process that used only two colours: blue-green and red-orange. It required two glass plate negatives, one made using a panchromatic emulsion and a red filter, the other made using an emulsion insensitive to red light. The two plates could be exposed as a "bipack" (sandwiched emulsion to emulsion, with a very thin red filter layer between), which eliminated the need for multiple exposures or a special color camera. After development, the silver images were bleached out with chemistry that hardened the bleached portions of the gelatin.