The seaweed Hypnea valentiae was extracted with solvents hexane, acetone and methanol. In the preliminary screening, hexane extract showed wide spectrum activity against ten human bacterial pathogens. The maximum level inhibition was observed against Escherichia coli, Vibrio parahaemolyticus and Micrococcus luteus and minimum against Vibrio cholerae. The assessment of seasonal variation in antibacterial activity with hexane extract showed higher activity during postmonsoon season followed by summer, premonsoon and monsoon season. The epiphytic bacterial density on surface of Hypnea valentiae was 25 x 103 CFU/cm2. Ten epiphytic bacteria (HV1- HV10) was isolated and the strain HV10 showed higher activity which was identified based on the morphologically and biochemical character as Alcaligenes sp. It was further cultured and the culture supernatant was extracted with hexane, n-butanol and water. The n-butanol extract showed higher activity against Shigella flexneri and minimum against Escherichia coli, Pseudomonas aeruginosa and Streptococcus epidermis. The investigation has proved that seaweed Hypnea valentiae and its epiphytic bacterial strain could be the potential source of bioactive compounds of biomedical importance.