The intensive aquaculture industry has recently gained popularity among animal producers due to its low costs and great efficiency. However, disease outbreaks have seriously threatened the industry's growth. Conventional disease prevention and treatment options, such as vaccines and medications, face significant challenges and limitations. Thus, finding natural feed additives as cost-effective alternatives to existing disease-fighting methods is important for sustaining ecologically friendly aquaculture. Cinnamon (Cinnamomum sp.) is considered one of the most promising feed additives, as established by its beneficial effects on numerous fish species. Cinnamon contains a high concentration of bioactive compounds, including essential oils (cinnamaldehyde and cinnamic aldehyde), tannins, polyphenols, saponins, carbohydrates, and flavonoids. Most of these bioactive have roles as enzyme modulators, redox-active transition metal chelators, and scavengers for nitrogen and reactive oxygen species. These compound activities confer cinnamon a wide range of biological benefits, including antimicrobial, antiinflammatory, and antioxidant properties. Therefore, cinnamon or any of its derivatives are becoming valuable as a natural feed additive for fish because of their beneficial effects on feed utilization, fish growth, and resistance to pathogens. This chapter attempts to research the effects of cinnamon dietary supplementation on fish growth and health, with a special emphasis on the immunostimulant activity of cinnamon and its safety aspects.