Biofouling or biological fouling is the undesirable accumulation of micro organism, plants, algae and/or animals on wetted structures.

Biofouling is especially economically significant on ships' hulls where high levels of fouling can reduce the performance of the vessel and increase its fuel requirements. Biofouling is also found in almost all circumstances where water based liquids are in contact with other materials. Industrially important examples include membrane systems, such as membrane bioreactor and  reverse osmosis spiral wound membranes cooling water cycles of large industrial equipments and  power station. Biofouling can also occur in oil pipelines carrying oils with entrained water especially those carrying used oils, cutting oils, soluble oil or hydraulic oils

Anti-fouling is the process of removing the accumulation, or preventing its accumulation. In  industrial processes bio-dispersants can be used to control biofouling.

There are two types of Biofouling , is divided into microfouling — biofilm formation and bacterial adhesion — and macrofouling — attachment of larger organisms, of which the main culprits are bamacles , mussels,  polycheate worms, bryozoans, and  seaweed. Together, these organisms form a fouling community.

Individually small, accumulated biofoulers can form enormous masses that severely diminish ships' maneuverability and carrying capacity. Fouling causes huge material and economic costs in maintenance of mariculture, shipping industries, naval vessels, and seawater pipelines. Governments and industry spend more than US$ 5.7 billion annually to prevent and control marine biofouling.