To investigate the antibacterial components from Sagittaria trifolia var. sinensis. The separation was performed over silica gel and Sephadex LH-20 column. The structures of the obtained compounds were determined on the basis of NMR data and chemical methods. Ten diterpenoids were isolated. Sagittine H (1), sclareol (2), and 19-β-L-3′-acetoxyarabinofuranosyl- ent-kaur-16-ene-19-oate (3) were evaluated for their abilities to inhibit the growth of three oral pathogens, Streptococcus mutans ATCC 25175, Actinomyces naeslundiis ATCC 12104, and Actinobacillus actinomycetemcomitans ATCC 43717, using minimum inhibitory concentration (MIC) assays. sagittine H (1) is a new ent-rosane glycoside and demonstrated MIC of 62.5 μg·mL −1 against both S. mutans and A. naeslundiis. Compound 3 exhibits strong activity against S. mutans and A. naeslundiis with MIC of 15.6 μg·mL −1 for both.