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This study investigated the impact of dietary guava leaf extract (GLE) on immunity, resistance to Pseudomonas aeruginosainfection, and growth in Oreochromis niloticus(Nile tilapia). Four groups of fish (28.6 g.fish−1) were fed diets supplemented with ethanolic GLE at 0 (control), 3, 5, or 7 g.fish−1diet (GLE0diet, GLE3diet, GLE5diet, GLE7diet) at 28 °C. Fish were fed thrice daily at 15 g.kg−0.8.d−1for 42 days. Innate immune parameters, including mucus-bactericidal activity (MBA), serum-bactericidal activity (SBA), bacterial agglutination activity (BAA), and phagocytic activity (PA), were assessed on days 14, 28, and 42. Weight gain (WG), specific growth rate (SGR), and feed conversion ratio (FCR) were measured on day 42. Post-feeding, fish were injected with P. aeruginosa, and mortality was recorded for 30 days. Results showed that GLE7dietsignificantly increased MBA, SBA, BAA, and PA by 1.8-, 3.3-, 4.2-, and 3.5-fold, respectively, compared to control. GLE7dietalso showed the highest significant reduction in post-challenge mortality (21 %) compared to the control group (83 %). Growth metrics for GLE7dietshowed WG of 43.8 g, SGR of 2.21%.d−1, and FCR of 1.19, compared to WG of 37.1 g, SGR of 1.98%.d−1, and FCR of 1.41 in the control group. SGR showed an exponential increase (R² = 0.935) and FCR a decrease (R² = 0.958) with increasing GLE levels. GLE5dietand GLE7dietsignificantly increased MBA at all-time points compared to control. GLE7dietsignificantly increased SBA on days 14, 28, and 42 compared to control. GLE7dietsignificantly enhanced BAA at all-time points. GLE5dietand GLE7dietsignificantly increased PA at all time points compared to control. GLE7dietled to increased expression of IL-1, IL-6, IFN-α, IFN-β, and TNF-α genes, particularly from 10 to 30 days post-infection. Dietary GLE at 7 g.kg−1significantly enhanced innate and acquired immune responses, growth performance, and resistance to P. aeruginosain Nile tilapia. This study recommended incorporating 7 g.kg−1GLE in Nile tilapia diets to improve disease resistance and growth performance.