In biology, detritus is non-living particules of organic origin. Typically, it includes the bodies or fragments of dead organisms as well as faecal material.

Detritus is typically colonized by communities of fungi and microorganisms which act to decompose (break down) the material. In terrestrial ecosystems, leaf litter and other organic matter gets mixed with soil. The result is called humus. Detritus of aquatic ecosystems is organic material suspended in water, which is sometimes referred to as marine snow. Marine snow is the basis of much life on the ocean floor. It is the food of animals which operate as 'filter-feeders'.