The current study investigated the protective role of Chlorella vulgaris against Aeromonas hydrophila infection in Nile tilapia. A total of 135 Orechromus niloticus fish were divided into three groups in tri-replicates. The first control group was fed a basal diet, the second group (Ch 3%) fed a diet enriched with 3% C. vulgaris, and the last group (Ch 6%) fed a 6% C. vulgaris diet. Along with the feeding trial, the hematological, biochemical, immunological, and histopathological parameters were comparatively evaluated, and after 6 weeks, the challenge against A. hydrophila was conducted. The erythrocyte count and Hb level remained unchanged, while the leukogram profile, serum lysozyme activity, and respiratory burst activity showed significant upregulation in Chlorella fed groups compared with the control group. In addition, the levels of hepato-renal biomarkers were significantly unchanged compared to the control group. Furthermore, the lower Chlorella diet showed prominent effects on maintaining these biomarkers. The survival rate was estimated after the challenge with A. hydrophila. Chlorella-treated fish groups, Ch6% and Ch3%, exhibited a better survival rate of 100% and 93.33%, respectively, compared with the control (6.67%). In conclusion, this study recommended incorporating C. vulgaris into tilapia diets to improve their immunity and disease resistance power.