Of 44 species of seaweed screened for potential anti-Gardnerella vaginalis activity, 27 (61.4%) showed antimicrobial activity by the agar disk-diffusion method. Among them, the strongest activities against the pathogen were exhibited by Chlorophyta, with Ulva pertusa producing an 11.3-mm zone of inhibition at 5 mg disk⁻¹. The MIC values of U. pertusa extracts against both G. vaginalis KCTC 5096 and KCTC 5097, the main cause of vaginosis, were 312 μg ml⁻¹, while the MIC values against both Candida albicans KCTC 7270 and KCTC 7965, the main cause of candidiasis, were 2.5 mg ml⁻¹. Against Lactobacillus gasseri KCTC 3173 and Lactobacillus jensenii KCTC 5194, members of the normal vaginal microflora, no inhibitory effect was seen even at 10 mg ml⁻¹. To identify the primary active compounds, a U. pertusa powder was successively fractionated according to polarity, and the main active agents against G. vaginalis KCTC 5096 were determined to be nitrogenous compounds (156 μg ml⁻¹ of the MIC value). According to these results, it was suggested that extracts of the seaweed U. pertusa are valuable for the development of natural therapeutic agents for treating women with bacterial vaginosis.