Periodontitis is one of oral infectious diseases which leads to alveolar bone destruction and ultimately possible tooth loss. Porphyromonas gingivalis (Pg) has been reported strongly associated with the progression of this disease. Many researches have proposed Morinda citrifolia (MC) juice to possess many biologically significant activities including antibacterial activity, but its effects on Pg is still unknown and uninvestigated. In this study, we examined the MC juice abilities of inhibiting Pg growth and adherence. We deduced the minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of MC fermented liquid against two strains of Pg by broth microdilution method. Our results showed both the MIC of MC fermented liquid against Pg ATCC 33277 were 0.25x concentration of original MC fermented liquid and the MIC against Pg A7436 was 1x. Both 2x and 3x MC fermented liquid showed clear bacterial inhibition zones against Pg ATCC 33277 in agar diffusion test but not significant against Pg A7436. We also inoculated Pg on the 96-well plates to determine the MC fermented liquid effects on Pg adherence. We found both 1x and 2x MC fermented liquid inhibited Pg ATCC 33277 adhesion better than control. Finally, we co-cultured MC fermented liquid with Pg for various time points to test the ability of MC fermented liquid to inhibit the growth of Pg. The data suggested that 2x concentrated MC fermented liquid can inhibit Pg growth at least for 24 hours. We concluded that MC fermented liquid featured a good antibacterial activity against Pg, a major periodontal microorganism, and as a result, the frequent usage of MC fermented liquid may reduce periodontitis progression.