The effect of 0%, 0.01%, 0.1%, and 1.0% supplementation of diets with Alnus firma on haematological and innate immune parameters in olive flounder (Paralichthys olivaceus) against Tenacibaculum maritimum infection was investigated. The supplemented diets were given for 30 d and then the fish were infected with T. maritimum. The particular diets were continued for 1, 2, and 4 weeks after the infection. The white blood cell count did not significantly change after any diet in the first week, whereas in 2nd and 4th week it was significantly increased in fish fed 0.1% and 1.0% supplementation diets. The number of red blood cells did not significantly change in fish fed 0.01% and 0.1% supplementation diets in any weeks but it was significantly increased in 1.0% supplementation diet. The haemoglobin and haematocrit levels did not significantly change after any diet in the first week; however, significantly increased in fish fed 0.1%, and 1.0% supplementation diets for 2 and 4 weeks. The biochemical parameters such as total protein and cholesterol levels significantly increased in fish fed 0.1% and 1.0% supplementation diets in the 2nd and 4th week, whereas in case of calcium levels the changes were observed from weeks 1 to 4. The glucose level significantly increased in all diets in weeks 2 and 4. The respiratory burst activity and lysozyme activity significantly were enhanced in 0.1% and 1.0% supplementation diets from weeks 1 to 4 compared to 0% diet. The cumulative mortality was lower in groups fed 0.1% and 1.0% supplemented diets than in the 0.01% diet. This study reported that 0.1% and 1.0% Alnus firma-supplemented diet protected the olive flounder against T. maritimum infection due to positive haematological, biochemical, and innate immune parameter changes.