A genetically modified organism (GMO) is an organism whose genetic material has been altered using genetic engineering techniques. Organisms that have been genetically modified include micro-organisms such as bacteria and yeast, plants, fish, and mammals. GMOs are the source of genetically modified foods, and are also widely used in scientific research and to produce useful goods other than food.

The article focuses on what organisms have been genetically engineered, and for what purposes. The article on genetic engineering focuses on the history and methods of genetic engineering, and on applications of genetic engineering and of GMOs. The two articles cover much of the same ground but with different organizations (sorted by organism in this article; sorted by application in the other). There are separate articles on genetically modified crops, genetically modified food, regulation of the release of genetic modified organisms, and controversies.