A 67-year-old man presented with a rare case of cavernous sinus thrombophlebitis (CST) caused by Porphyromonas gingivalis with abscess formation extending to the orbital cavity. Neuroimaging demonstrated a cystic lesion in the right cavernous sinus that was hyperintense on diffusion-weighted imaging. The patient was successfully treated with surgical drainage and antibiotic administration. CST is rare and often has a fulminant progression with high rates of morbidity and mortality. The differential diagnosis of cavernous sinus lesions should include CST. Early recognition and differentiation from other diseases with aggressive medical and possible surgical intervention are necessary to reduce mortality and long-term sequelae. Diffusion-weighted imaging is useful for the early recognition and differentiation of CST from other diseases.