We investigated the regulatory effects of recombinant growth hormone (rGH) on growth and immunity in an ornamental seawater fish species, the cinnamon clownfish (Amphiprion melanopus). Cinnamon clownfish received intraperitoneal injections of rGH or saline at doses of 0.1 or 1 µg/g, weekly, for 12 weeks. At the end of the experiment, the rGH injection group exhibited the greatest total length (5.3±0.3 and 5.6±0.2 cm, respectively, for the two doses). Moreover, after injection of rGH, the expression levels of pituitary GH mRNA and liver insulin-like growth factor I (IGF-I) mRNA increased significantly (by 18.9-fold and 5.7-fold, respectively), as assessed by quantitative real-time polymerase chain reaction, while the splenic levels of immunoglobulin M protein increased significantly (by 10.5-fold) as assessed by Western blot analysis. Plasma lysozyme and melatonin levels also increased by 2.1- and 1.37-fold, respectively, over those of the control group. Plasma alanine and aspartate aminotransferase levels at 12 weeks (764±46.4 and 3002±101.2 IU/l, respectively) were significantly higher than those at baseline; however, treatment with rGH decreased these values at 12 weeks (276±30.4 and 600.5±59.2 IU/l, respectively). These findings support the hypothesis that rGH plays important roles in the regulation of the GH–IGF-I axis for growth and immunity in the cinnamon clownfish.