A 56-year-old female visited our dermatology clinic with a two-month history of persistent painful oral mucositis and erythematous to violaceous papules and plaques without blisters over her trunk and limbs. The patient had been treated with multiple drugs including high-dose oral corticosteroid and ciclosproin under the clinical impression of a generalized lichen planus; however the disease remained inadequately controlled for four months. Paraneoplastic pemphigus (PNP) was considered due to therapy-refractory erosive mucosal lesions, weight loss, and recent hemoptysis. Two biopsies of the skin and oral mucosa revealed interface dermatitis with a thick lichenoid inflammatory infiltrate. Direct immunofluorescence on skin lesion revealed intraepidermal, intercellular IgG staining. Circulating antibodies against desmogleins 1 and 3 were negative in ELISA analysis. Serum indirect immunofluorescence using rat bladder as a substrate was positive for intraepithelial binding. Subsequently computed tomography of the thorax revealed an invasive mediastinal mass and the final diagnosis of PNP associated with malignant thymoma was made. PNP is a debilitating chronic blistering mucocutaneous disease associated with an underlying neoplasm and a poor prognosis. When treatment-recalcitrant erosive mucosal lesions persist, even in patients without known malignancy, PNP should be ruled out.