This research investigated the antibacterial efficacy of ethanolic extract from the brown seaweed Hormophysa cuneiformis against Aeromonas eucrenophila, A. hydrophila, and A. taiwanensis at three concentrations: 0.10, 0.20, and 0.40mg/ mL. The agar disc diffusion method was employed to assess antimicrobial activity. Results indicated that H. cuneiformis extracts did not exhibit any antibacterial activity against A. eucrenophila at the tested concentrations, suggesting no inhibitory effect. Conversely, notable antibacterial effects were seen against A. hydrophilaand A. taiwanensis, with the most substantial zones of inhibition measured at 0.10mg/ mL (18.42±1.27mm) and 0.20mg/ mL (24.03±0.56mm), respectively. Varying levels of antibacterial activity were recorded for the control antibiotic, with the highest inhibition zone for A. hydrophila(18.42±1.27mm). The findings suggest that the bioactive compounds in the extracts have an optimal range for antibacterial efficacy. Further studies are recommended to explore the mechanisms of inhibition and the potential use of this extract as antimicrobial agent in aquaculture.