High mobility group box 1 (HMGB1) is a non-histone DNA-binding protein that is secreted into the extracellular milieu in response to inflammatory stimuli. The secreted HMGB1 mediates various inflammatory diseases, including periodontitis; however, the underlying mechanisms of HMGB1-induced periodontal inflammation are not completely understood. Here, we examined whether anti-HMGB1 neutralizing antibody inhibits periodontal progression and investigated the molecular pathology of HMGB1 in vitro and in vivo. In vitro analysis indicated that HMGB1, granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-1β (IL-1β) were secreted in response to tumor necrosis factor-α (TNF-α) stimuli in human gingival epithelial cells (HGECs) and human monocytic leukemia cells (THP-1) treated with phorbol myristate acetate. Increased levels of GM-CSF and IL-1β were observed in the conditioned media from TNF-α-stimulated HGECs and THP-1 in vitro . Simultaneous stimulation with TNF-α and anti-HMGB1 antibody significantly decreased TNF-α-induced inflammatory cytokine secretion. Experimental periodontitis was induced in mice using Porphyromonas gingivalis -soaked ligatures. The extracellular translocation was confirmed in gingival epithelia in the periodontitis model mice by immunofluorescence analysis. Systemic administration of anti-HMGB1 neutralizing antibody significantly inhibited translocation of HMGB1. The anti-HMGB1 antibody inhibited periodontal inflammation, expression of IL-1β and C-X-C motif chemokine ligand 1 (CXCL1), migration of neutrophils, and bone resorption, shown by bioluminescence imaging of myeloperoxidase activity, quantitative reverse transcription-PCR (RT-PCR), and micro-computed tomography analysis. These findings indicate that HMGB1 is secreted in response to inflammatory stimuli caused by periodontal infection, which is crucial for the initiation of periodontitis, and the anti-HMGB1 antibody attenuates the secretion of a series of inflammatory cytokines, consequently suppressing the progression of periodontitis.