A heterotroph (/ˈhɛtərɵtroʊf/; ἕτερος heteros = "another", "different" and τροφή trophe = "nutrition") is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight (photoautotrophs) or inorganic compounds (lithoautotrophs) to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon dioxide. These reduced carbon compounds can be used as an energy source by the autotroph and provide the energy in food consumed by heterotrophs.

Heterotrophs can be divided into two broad classes: photoheterotrophs and chemoheterotrophs. Photoheterotrophs, including most purple bacteria and green bacteria, produce ATP from light and use organic compounds to build structures. They consume little or none of the energy produced during photosynthesis to reduce NADP⁺ to NADPH for use in the Calvin cycle, as they do not need to use the Calvin cycle if carbohydrates are available in their diets. Chemoheterotrophs produce ATP by oxidizing chemical substances. There are two types of chemoheterotrophs: chemoorganoheterotrophs and chemolithoheterotrophs.