Diabetic patients are at an increased risk of developing severe and progressive periodontitis. Periodontal disease also increases the severity of diabetes by enhancing insulin resistance. Therefore, the regulation of periodontal inflammation in diabetic patients may contribute to the control of both diseases. Glycyrrhizic acid exerts anti-inflammatory effects by inhibiting high mobility group box 1 (HMGB1). HMGB1, one of the ligands of the receptor for advanced glycation end products (RAGE), is a damage-associated molecular pattern and induces inflammatory cytokine production. In the present study, we examined the effects of glycyrrhizic acid on ligature- and Porphyromonas gulae infection-induced periodontitis as well as the involvement of the HMGB1-RAGE axis in diabetic model mice. The molars of diabetic model mice, established by feeding HFD32 to KK/TaJcl mice, were subjected to silk thread ligation and P. gulae was then intraorally applied in the presence or absence of glycyrrhizic acid given topically. The topical application of glycyrrhizic acid suppressed ligature/P. gulae-induced increases in interleukin (IL)-6 and tumor necrosis factor (TNF)-α at the mRNA level in the gingiva and at the protein level in serum. Furthermore, glycyrrhizic acid suppressed ligature/P. gulae-induced increases in serum amyloid A (SAA) in serum and fasting blood glucose levels. It also suppressed ligature/P. gulae-induced increases of HMGB1 and RAGE at the mRNA level in the gingiva and at the protein level in serum. A mouse anti-HMGB1-neutralizing antibody inhibited increases in serum glucose levels. In conclusion, topical treatments with glycyrrhizic acid may suppress periodontal and systemic inflammation and reduce blood glucose levels through the HMGB1-RAGE axis in diabetic mice.