Periodontal disease is a serious problem in the veterinary field, as it is reported that more than 80% of dogs over 6 years of age suffer from periodontal disease. Severe periodontal infection is irreversible; therefore, once the supporting tissues are damaged, there is no possibility of recovery. Therefore, preventive dentistry, such as daily tooth-cleaning and dental gel from early life, at the veterinary hospital as well as at home, is quite important. In this study, we focused on persimmon tannin, a polyphenol extracted from persimmon, and examined the bactericidal, anti-halitosis and anti-inflammatory effects in vitro using Porphyromonas gulae (P. gulae), which is a major contributor to the progression of periodontal disease in dogs. Clinical study in 20 dogs with severe periodontal disease was also conducted by daily oral application of 1% persimmon tannin gel. Whereas persimmon tannins did not alter the growth of P. gulae, significant inhibition of CH₃SH production by P. gulae, and significant inhibition of IL-6, IL-1β and TNFα production by mouse macrophage cell line infected with P. gulae were observed in persimmon tannin treated group. In vitro anti-halitosis, and anti-inflammatory effects of persimmon tannin were confirmed by clinical experiments in dogs with P. gulae-associated periodontal diseases, and one-month oral treatment with 1% persimmon tannin contained dental gel significantly reduced halitosis and P. gulae activity. Our findings suggest that oral treatment with persimmon tannin can be a preventive option for periodontal disease in dogs.