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Introduction Extramammary Paget's disease is an eczematous skin condition that affects the vulva and perineum. Extramammary Paget's disease secondary to urothelial carcinoma is a rare condition that is typically treated with invasive surgical resection of the lesion. Case presentation An 80‐year‐old woman with a 7‐year history of urothelial carcinoma presented with erythema of the labia majora. Immunostaining of skin biopsy specimens suggested extramammary Paget's disease secondary to urothelial carcinoma. The patient did not consent to resection of the lesion. Nine cycles of first‐line platinum‐based chemotherapy for metastatic urothelial carcinoma were administered. As tumor cells remained after systemic chemotherapy, pembrolizumab will be administered to the patient for treating residual extramammary Paget's disease. Conclusion Platinum‐based chemotherapy can control extramammary Paget's disease secondary to urothelial carcinoma.