We report a case of septic shock treated with PMX-DHP that was complicated with Lemierre syndrome caused by Fusobacterium necrophorum. The patient was a 31-year-old man who was initially diagnosed with influenza. He received treatment; however, because his symptoms gradually worsened, he was transported to our hospital 10 days following his diagnosis. His initial examination revealed symptoms of respiratory distress and an altered level of consciousness. Based on laboratory and imaging results, it became clear that he suffered from septic shock of unknown etiology, disseminated intravascular coagulation, and acute renal and respiratory failure. We initiated treatment with vasopressors, an antibacterial agent, immunoglobulins as well as an appropriate ventilator management; however, his unstable circulatory condition continued. As soon as PMX-DHP was initiated, 2 days following admission to the ICU, his circulatory instability normalized. F. necrophorum was ultimately detected after a culture examination, and contrast-enhanced computed tomography revealed a jugular vein thrombus, which led to the diagnosis of Lemierre syndrome. The patient's condition gradually improved, and he was discharged from the ICU after 19 days.