Diffuse large B cell lymphoma (DLBCL) of the ethmoid sinus is very rare entity. The presenting symptoms are usually similar to those of benign inflammatory diseases. We report a case of a 50-year-old male patient who presented with acute onset progressive proptosis right periorbital swelling, redness and treated initially as periorbital cellulitis secondary to right maxillary and ethmoidal fungal sinusitis, but proven to have be a diffuse large B-cell lymphoma (DLBCL) of the ethmoid sinus. HIV serology was negative. The patient was referred to the department of oncology where chemotherapeutic treatment for the lymphoma. We emphasize the importance of considering primary lymphoma of the ethmoid sinus in the differential diagnosis of periorbital cellulitis and refractory rhinosinusitis, to help in early diagnosis, which can improve prognosis.