The present study aimed to investigate the beneficial effects of dietary rosemary leaf powder (RLP) as growth promotor and immunostimulant on plasma immunity as well as stress responses in common carp (Cyprinus carpio) fingerlings. In a 65-day feeding trial, fish were fed on experimental diets containing RLP (0, 1, 2 and 3%). At the end of the experiment, fish performance, survival rate, and proximate composition were investigated. Thereafter, blood samples were collected from treatments for investigation of biochemical and humoral immune parameters. Then, fish were subjected to net-confinement crowding stress for 3 h. Blood samples were collected before and after stress pressure, to measure plasma cortisol and glucose levels. The obtained results showed that fish growth increased along with the increase of dietary RLP levels up to 3%. But proximate composition indices were not influenced by dietary RLP treatments. The RLP-supplemented fish had significantly higher plasma total protein, albumin, globulin, lysozyme, and ACH50 compared to the control group. WBC values were significantly higher in the 2 and 3% RLP-treatments compared to those of the control one. Whereas, fish treated with 3% RLP showed highest plasma total Ig levels. CAT and SOD activities were significantly higher in RLP-supplemented groups than in the control group. Cortisol and glucose levels in all groups were significantly affected by stress; nevertheless, the RLP administration significantly mitigated the stress-induced increase in plasma cortisol and glucose levels. The present results confirmed that the oral administration of RLP has the potential to promote growth performance, enhance antioxidant and immunological parameters, and mitigate the negative effects of crowding stress on common carp fingerlings. © 2019 Elsevier B.V.