Squamous cell carcinoma in situ (SCCIS) is a precancerous lesion that usually appears as a well-demarcated erythematous scaly plaque, histopathologically characterized by atypical full-thickness disarrangement and pleomorphic atypical keratinocytes. Extramammary Paget disease (EMPD) is an intraepidermal tumor that usually affects skin in the genital region, histopathologically identified by large cells with pale and vacuolated cytoplasm, called Paget cells, in the epidermis. An 84-year-old female presented with an asymptomatic solitary scaly erythematous plaque on her back first noticed 10 years prior. A keratotic mass was noted immediately adjacent to the plaque. A biopsy was performed; one specimen showed atypical full-thickness disarrangement and pleomorphic keratinocytes with negative CK7 staining in the epidermis, and another specimen showed Paget cells above the basal layer with positive CK7 staining in the epidermis. Based on these findings, the patient was diagnosed with SCCIS coexisting with EMPD. The patient underwent wide surgical excision of the lesion with reconstruction, and is being monitored without recurrence. (Korean J Dermatol 2023;61(3):191∼193)