Antibiotic-resistant bacterial pathogens are an escalating threat to brackish-water aquaculture, driving interest in plant-derived antimicrobials as sustainable alternatives to synthetic drugs. This study evaluated the phytochemical composition and antibacterial potential of methanolic Couroupita guianensis (cannonball tree) bark extract against Aeromonas hydrophila and Vibrio harveyi isolated from the skin and gills of Tilapia (Oreochromis mossambicus) collected from the Retteri river, Chennai, India. Qualitative screening confirmed the presence of alkaloids, flavonoids, tannins, terpenoids, steroids, glycosides, amino acids, carbohydrates, phenolics and coumarins in the bark extract, while saponins, lignins and anthocyanins were absent. Quantitative assay showed a concentration-dependent increase in total flavonoid (77.3-186.1 µg quercetin equivalent) and total phenolic content (50.1-59.6 µg gallic acid equivalent). Both bacterial isolates were confirmed as Gram-negative rods by standard biochemical tests (triple sugar iron, oxidase, catalase, methyl red, urease). Disc diffusion assay demonstrated a dose-dependent, though modest, inhibitory effect of the extract (10-100 µg/mL) relative to the gentamicin positive control. Broth microdilution established a minimum inhibitory concentration (MIC) of 75 µg/mL for A. hydrophila and 100 µg/mL for V. harveyi. PCR amplification of the Tilapia Lake Virus (TiLV) segment from bacterial-isolate-associated gill and skin DNA yielded amplicons in the expected 400-500 bp range, consistent with reported TiLV detection protocols. These findings indicate that C. guianensis bark harbours a broad-spectrum phytochemical profile with measurable, dose-dependent antibacterial activity against two economically important brackish-water fish pathogens, supporting further investigation of the extract as a candidate eco-friendly phytotherapeutic for sustainable aquaculture disease management.