This study examined in vitro, the antibacterial activity of Malva parviflora leaves ethanolic extract against Pseudomonas fluorescens and its minimum inhibitory and minimum bactericidal-concentrations (MIC, MBC), identified its biological compounds responsible for its efficacy by Gas Chromatography-Mass Spectrometer (GC-MS) device besides evaluated its effect in vivo on Nile tilapia fish as a medicinal plant to combat P. fluorescens challenge. Two hundred twenty-five fishes were divided into five groups; G1: negative control. G2: positive control; infected at the 60 days (1x103CFU/0.2ml I/M). G3, G4 and G5 were fed with (0.25, 0.5 and 1 g/kg) respectively (60 days), then infected. On the thirty, sixty days and post challenge, blood samples were collected. Sensitivity test revealed the susceptibility of P. fluorescens to the extract; MIC and MBC were 3.90 and 31.25 mg/ml respectively. Body weight, phagocyte nitric oxide and lysozyme activity increased significantly in all treated groups than controls especially G4 while both G3 and G4 increased the survival rate to 80% compared to G2 (40%). Total leucocytic, lymphocyte (L) and monocyte (M) counts increased significantly in all treated groups than G1 throughout the study. Aspartate (AST) and alanine aminotransferases (ALT), alkaline phosphatase (ALP), urea and creatinine showed no significant difference between G3 and G1, while G4 and G5 increased in a dose-dependent manner. Total protein (TP), albumin and globulin revealed no significant difference among groups. Malondialdehyde (MDA) showed the best decrease in G3 than G1. Post challenge all treated groups elicited a significant decrease than G2. Superoxide dismutase (SOD) and reduced glutathione (GSH) were significantly higher in all treated groups than the controls in varying degrees. Finally, the used extract enhanced the fish performance, immune system and antioxidant biomarkers without any side effects on liver and kidney functions at 0.25 g/kg, while 0.5 g/kg gave the highest weight gain.