This work studied in vitro; the antibacterial activity of Vitex agnus-castus (VAC) extract against V. anguillarum, its minimal inhibitory and bactericidal concentrations (MIC and MBC) and detected its bioconstituents using gas chromatography-mass spectrometry (GC-MS) analysis besides evaluated its effect in vivo in carp either alone or combined with antibiotic to prevent or treat V. anguillarum infection. A total of 180 fish were divided into 6 groups; G1: negative-control, G2: V. anguillarum infected-control, G3: infected-(marbofloxacin (MAR) 10 mg/kg body weight), G4: infected-(VAC 1gm/kg ration), G5: infected-(VAC 1 gm/kg + MAR 10 mg/kg) all treatments lasted 7 days and G6: prophylactic treated-(VAC 1 gm/kg/30 day) then challenged. Results of scanning electron microscope revealed changes in bacterial morphology and loss of flagella by VAC where its MIC and MBC were 1.95 and 15.63 mg/ml respectively. Growth performance and survival rate improved in all treated groups in this descending order (G6, G4 then G5). All treatments significantly increased total protein, albumin, globulin, superoxide dismutase and decreased aspartate and alanine aminotransferases activities, malondialdehyde, urea and creatinine than G2 and mostly near to G1 in this ascending order (G3, G4 then G5). In G6, VAC protected carp from the infection and improved growth, survivability and most of blood parameters. MAR residue in fish musculature needed 4 days as a withdrawal period to be less than the maximum residue limits (MRL), while combination of VAC with MAR decreased it less than the MRL from the 1st day post treatment. Finally, VAC had a considerable antibacterial activity against V. anguillarum in both prevention and treatment assays. It can be used either alone or adding with MAR.