Although Dialister pneumosintes is a commensal microorganism of the oral cavity, it may sometimes cause severe systemic infections. We report a case of Lemierre syndrome (LS), which developed after D. pneumosintes infection, in a 73-year-old Japanese woman who was admitted to the hospital for throat pain, neck swelling, and fever for 3 days. She had a 3-month history of neglected dental caries, gingivitis, and periodontitis. Physical examination revealed right tonsillar erythema and swelling, and computed tomography (CT) showed peritonsillar and retropharyngeal abscesses. Ampicillin/sulbactam was promptly administered after collecting two sets of blood cultures. Surgical drainage for peritonsillar and retropharyngeal abscesses was also conducted on the second hospital day. Although only commensal oral microflora grew in the culture from the drained pus, Gram-negative bacilli were confirmed in the anaerobic blood cultures. Metronidazole was administered intravenously; however, the fever and neck swelling persisted. Repeat CT performed on the fifth hospital day revealed right internal jugular vein thrombosis, a known complication of tonsillitis and pharyngitis once the infection extends beyond the oropharynx. We diagnosed she had coexisting LS, and anticoagulant therapy was added to her treatment regimen. Her condition improved, and she was discharged after completing 3 weeks of antibiotics. Conventional methods failed to identify the isolated bacterium, and 16S rRNA sequencing ultimately identified it as D. pneumosintes. In a literature review of bacteremia due to D. pneumosintes, poor oral hygiene was considered a probable risk factor for invasive D. pneumosintes infection. We consider this to be the case in our patient who presented with dental caries, gingivitis, and periodontitis. In addition, all cases revealed that the 16S rRNA gene sequencing is useful for identifying this species. Although the diagnosis of LS by physical examination is difficult, physicians should always consider it as a potential complication of infections in the pharyngeal area.