Detritus is a term used in biology and ecology to refer to dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. It is present in terrestrial ecosystems as leaf litter and other organic matter that is intermixed with soil, and in aquatic ecosystems as organic substances that are suspended in the water and accumulate in depositions on the floor of the body of water. Detritus typically hosts communities of microorganisms that colonize and decompose it, and it is an important source of nutrients for many organisms.

In practical terms, the most important constituents of detritus are complex carbohydrates, which are persistent (difficult to break down), and the microorganisms which multiply using these absorb carbon from the detritus, and materials such as nitrogen and phosphorus from the water in their environment to synthesize the components of their own cells.

Detritus can also refer to waste material or trash, especially left after a particular event. However, in ecology, it refers to matter composed of leaves and other plant parts, animal remains, waste products, and other organic debris that falls onto the soil or into bodies of water from surrounding terrestrial communities.