Abstract: Background and Objective: Periodontal disease and dental caries are frequent oral illnesses. Both have an important impact on quality of life are of polymicrobial origin and progress slowly. Plants are a valuable resource in health systems in developing countries and a large part of traditional treatments involve the use of plant extracts or their active substances. The objective of this study is to determine the antimicrobial activity of the methanolic extract of Psacalium decompositum using the plate diffusion method in vitro and to determine the minimum inhibitory concentration of Porphyromonas gingivalis, Prevotella intermedia and Streptococcus mutans. Materials and Methods: The plant was collected and identified, Psacalium decompositum and a methanol extract was prepared by mashing. Phytochemical screening was also performed. The methanol extract was evaluted by the plate diffusion method using different microdilutions in broth against the ATCC strains Porphyromonas gingivalis, Prevotella intermedia and Streptococcus mutans. About 0.2% chlorhexidine was used as a positive control and 5% ethanol as a negative control. The nonparametric statistic, the Kruskal-Wallis test was used to compare the three treatments and the Mann-Whitney test for each pair of treatments to identify significant differences between them. Results: The minimum inhibitory concentration of the methanolic extract of P. decompositum was 500 μg mL–1 for P. gingivalis and P. intermedia and 700 μg mL–1 for S. mutans (p<0.002). The minimum inhibitory concentration for chlorhexidine (positive control) was 900 μg mL–1 for the three bacteria studied. Conclusion: The methanolic extract was active against cariogenic and periodontopathogenic bacteria and represents a natural alternative for the control of these microorganisms that produce oral diseases. The methanolic extract of P. decompositum has antimicrobial activity making it a natural alternative for the treatment of caries and periodontal disease.