The antibacterial activities of methanol extracts of 19 commercial herbal medicines were measured against the fish pathogens Vibrio ichthyoenteri and Streptococcus iniae, which cause several fish diseases. Rhus javanica showed the strongest antibacterial activity against V. ichthyoenteri and S. iniae. The methanol extract of R. javanica was extracted further using several organic solvents with different polarities. The extract from the ethyl acetate fraction showed strong activity against both fish pathogens. The minimum inhibitory concentration (MIC) of the R. javanica extract was 32 μg/mL for V. ichthyoenteri and 128 μg/mL for S. iniae. Further purification and isolation of the active compound (s) responsible for these activities and further study of the synergistic effect using combinations of antibiotics against pathogenic bacteria are needed.