BACKGROUND Dialister pneumosintes is a suspected periodontal pathogen. It can affect different parts of the body either by hematogenous transmission or regional spread. Here, we report a case of 30-year-old previously healthy woman diagnosed with mediastinal and neck abscess caused by this pathogen. CASE REPORT A 30-year-old woman presented with a 1-day history of fever, vomiting, and diarrhea. She was on her last dose of a 2-week course of oral antibiotic for suspected dental abscess. On admission, parenteral broad-spectrum antibiotic was started for sepsis of unknown source. Because of intermittent spike of high temperature despite being on an antibiotic, cross-sectional imaging was performed, which revealed a superior mediastinal abscess with extension in the neck. She was referred to the ENT surgeon for incision and drainage of the collection. However, the procedure was complicated by injury to the right internal jugular vein. Her postoperative period was also convoluted with the development of pulmonary embolism, followed by deep vein thrombosis of the right upper limb. Her pus polymerase chain reaction test detected 16s rRNA gene, suggestive of gram-negative anaerobic bacilli, and anaerobic blood culture grew Dialister pneumosintes. After a prolonged course of illness and antibiotic treatment, she recovered well, and now is back to her normal activities. CONCLUSIONS Potential life-threatening complications may develop from periodontal infection by this microorganism. In patients being treated for sepsis of unknown origin, not responding to antibiotic treatment, and with a history of recent periodontal infection, a deep-seated abscess needs to be considered.