Rhinosinusitis is a common disease in otorhinolaryngologic clinics. It is diagnosed and treated by chief complaints and physical examinations. A 19-year-old female patient presented with left rhinorrhea, sneezing, itchy throat, cough and left frontal headache for over 1 month. Unilateral subacute rhinosinusitis was impressed, and medicine failed to cure her symptoms at other clinics. However, unilateral nasal leech infestation was identified at our clinic. After the nasal cavity was locally anesthetized, the worm was removed under endoscopy. Herein the rare case is reported as a warning that we should take more time to evaluate a nasal lesion, and a nasal leech infestation will not be misdiagnosed as unilateral rhinosinusitis.