A defoamer or an anti-foaming agent is a chemical additive that reduces and hinders the formation of foam in industrial process liquids. The terms anti-foam agent and defoamer are often used interchangeably.

A defoamer is normally used in industrial processes to increase speed and reduce other problems. It addresses both problems with surface foam and entrained or entrapped air.
Generally a defoamer is insoluble in the foaming medium and has surface active properties. An essential feature of a defoamer product is a low viscosity and a facility to spread rapidly on foamy surfaces. It has affinity to the air-liquid surface where it destabilizes the foam lamellas. This causes rupture of the air bubbles and breakdown of surface foam. Entrained air bubbles are agglomerated, and the larger bubbles rise to the surface of the bulk liquid more quickly.

In many industrial processes, surface-active substances are employed in order to obtain desired effects. For instance, aqueous coating compositions require a range of auxiliary products and additives, such as emulsifiers to disperse water-insoluble binders, and additives to improve substrate wetting and pigment dispersion. An undesirable side effect of these surface-active substances however, is that they stabilize foam introduced in the course of preparation or application of the coating composition.

When talking about foam one has to distinguish between macrofoam and microfoam. Macrofoam is mostly located on the coating surface and is surrounded by a duplex film with two liquid/air interfaces (double layer), whereas microfoam occurs inside of a coating film (air entrapment) and is characterized by a single liquid/air interface. These two types of foam also differentiate defoamers from deaerators. Defoamers are mostly effective against macrofoam, whereas deaerators suppress microfoam. In practice, the terms "defoamer" and "deaerator" are often mixed up, and several commercial products are optimized to prevent macro- as well as microfoam.

Both kinds of foam impair surface optics of coatings, causing surface irregularities and reducing gloss and transparency. Microfoam also adversely affects the protective properties of coatings, as the effective film thickness is reduced and pinholes may be formed out of small bubbles.

The function of defoamers is based on disturbance of the double layer of the macrofoam lamella. Substances with very low surface tension are used, as they cannot be wetted by the foam bubble. Foam-stabilizing substances move away from the defoamer droplet, which finally causes collapse of the bubble. Surfactants are often used together with defoaming substances to improve the spreading behavior of the defoamer droplet on the bubble surface.