The aim of present study was to evaluate the effects of edible eryngii mushroom powder, Pleurotus eryngii (PE), for 63 days on haematological parameters, the serum immune responses, skin mucus, bactericidal activity, stress resistance, growth performance and digestive enzyme activities of Koi carp fingerlings (Cyprinus carpio koi). Fish were divided into five groups and each group was fed with dietary PE with five graded levels (0, 0.5, 1, 1.5 and 2%). The results showed a significant dose-dependent increase of Ht, Hb, MCV and MCH levels in fish fed dietary PE (P < 0.05). The highest levels of WBCs, lymphocytes and monocytes were measured in fish fed 1.5% and 2% of dietary PE (P < 0.05). The activities of total IG, lysozyme, Alternative haemolytic complement activity in serum of fish fed with 2% of dietary PE for 63 days as well as 5-min air exposure challenge test were significantly higher than other groups (P < 0.05). The most bactericidal activity was observed in skin mucus of fish fed with 1.5% of dietary PE against Streptococcus iniae (P < 0.05). The highest ratio of the lactobacillus count to the total viable count was observed in fish fed 2% of dietary PE. The α-amylase activity of fish fed with dietary PE (1, 1.5 and 2%) were significantly higher than control group (P < 0.05). Feeding fish 2% of dietary PE increased the trypsin and lipase activity compared to others groups (P < 0.05). The growth performance of fish fed 1.5% of dietary PE improved compared to control group (P < 0.05). The results revealed that feeding koi fish with dietary supplementation of PE (1.5 and 2%) improved the selected humoral innate immune responses, bactericidal activity of skin mucus and growth performance of koi fish.