Damselfish can refer to any of the following:

• The family Pomacentridae, comprising about 350 species of marine fish;
• All members of that family other than the Clownfishes
• One of a number of species belonging to the family Pomacentridae, most usually Chromis chromis. Many other species within the family have common names that include the word "damselfish", but in almost all cases this is qualified with an adjective or other descriptor.

The second of these senses is probably the most common. The average size of such damselfish is around 7.6 centimetres (3 inches). They are all marine and usually have bright colours. Some species of damselfish are able to adapt well in an average aquarium, but others such as the white-spotted damselfish cannot. Sometimes the fish cannot breed in captivity. The diet of a damselfish can include small crustaceans, plankton, and algae.

Many species of damselfish are kept as aquaria, and live in tropical coral reefs; however, many also live in temperate climates. One example would be damsels inhabiting the Catalina islands in California.

A common practice for Damselfish is as a biological stabilizer in new aquariums. The fish would live in the aquarium during its initial existence, and be used to allow the aquarium to biologically stabilize with beneficial bacteria. This practice is viewed negatively by many aquarists because of the foul conditions the fish are subjected to and the fact that there exist more humane methods to stabilize an aquarium.