Aquaculture aims to provide stable environmental conditions for fish to be able to grow and protect it from stress and diseases. In order to guarantee a sustainable yield of fish species, well-balanced diets should be considered. For this purpose, diets supplemented with feed additives have won the interest of researchers who are concerned with the aquaculture field, the most important of which are chitosan and chitosan nanoparticles. Chitosan [β-(1-4)-N-acetyl-D-glucosamine] is a cationic biopolymer, derived from the alkaline deacetylation of chitin; a natural polymer found in the exoskeletons of insects, crustaceans and fungal cell walls. Chitosan nanoparticles were synthesized either by chemical or physical cross linking. Chitosan nanoparticles are favored due to their large surface area, bioavailability and deep penetration to the target sites. Chitosan and chitosan nanoparticles have several unique biological properties that enable them to act as promising materials in variable applications, including biosafety, biocompatibility, improved solubility and biodegradability. The aforementioned properties enabled them to be used in several applications in fish aquaculture, such as enhancing growth performance and immune functions. Moreover, they act as safe feed additives, drug carriers and are widely used in water treatment as well. The applications of chitosan and chitosan nanoparticles in fish aquaculture attracted the interest of many researchers. The present review covered all information concerning the effect of chitosan and chitosan nanoparticles on fish, including antioxidant activity, growth enhancement and immunostimulatory effects. The discussed studies in the current review proved that chitosan and chitosan nanoparticles are regarded as promising materials with wide applications in fish aquaculture.