Introduction: Lichen planus pemphigoides (LPP) is a rare acquired autoimmune disease characterised by the evolution of subepidermal blisters on normal and lichen planus affected skin. Case summary: We describe a case of lichen planus pemphigoides in a 54-year-old Chinese woman. The patient presented initially with scaly psoriasiform plaques and was diagnosed to have guttate psoriasis. She was treated with narrowband ultraviolet (NBUVB) therapy twice weekly. Within a month of starting phototherapy, she experienced a flare up of her skin lesions with a generalised eruption of violaceous papules, tense bullae over the lower limbs as well as Wickham’s striae over the buccal mucosa. Histology of the violaceous papule over abdomen revealed interface dermatitis, while the specimen from a blister showed subepidermal bulla with linear deposition of IgG and C3 along the basement membrane zone. A diagnosis of LPP was made on clinicopathological grounds. The patient subsequently responded well to oral prednisolone at a dose of 0.5 mg/kg/day. Conclusion: This is the first case report of NBUVB alone unmasking LPP. In the presentation, we will describe the pathological mechanism of NBUVB in the development of LPP and the key features distinguishing LPP from bullous lichen planus (BLP), psoriasis and bullous pemphigoid (BP).