Lichen sclerosus or lichen sclerosus et atrophicus is a chronic inflammatory disease predominantly affecting the genital mucosa and skin. Clinically, it is characterized by white atrophic plaques in the anogenital region. The lesions are generally asymptomatic, but may cause discomfort with itching and pain. Extragenital mucosal involvement is very unusual, and lesions limited to the oral mucosa are even less frequent. Knowledge of such lesions is important in order to establish a differential diagnosis with other white oral lesions, and histological confirmation is required. We present the case of a 31-year-old woman with a well delimited, pearly white lesion located in the upper gingival mucosa, lip mucosa and adjacent skin. The lesion had led to loss of periodontal attachment of the affected tooth (2.3), causing pain in response to tooth brushing. The biopsy confirmed lichen sclerosus, and treatment was provided in the form of intralesional corticoid injections, followed by improvement of the mucosal lesion, though without recovery of the periodontal loss.