An 86-year-old man presented with pruritic, multiple, irregularly shaped, erythematous to brownish patches on the back for 3 weeks. He had been diagnosed with prostate cancer a year and a half ago and was on chemotherapy. A skin biopsy of the lesion revealed interstitial histiocytic infiltration throughout the dermis with few eosinophils and neutrophils, suggesting the diagnosis of interstitial granulomatous dermatitis (IGD). After one-month administration of systemic corticosteroids, the skin lesions markedly improved. Interstitial granulomatous dermatitis (IGD) is a rare dermatosis histologically characterized by an interstitial granulomatous infiltration. Clinically, it presents as erythematous to violaceous patches or plaques symmetrically located on the upper trunk and proximal limbs. The pathogenesis of IGD is unclear. However, since almost all cases of IGD are associated with an underlying inflammatory condition, it has been suggested to be a nonspecific sign of immune dysfunction. IGD has been associated with inflammatory arthritis, various medications, connective tissue diseases, autoimmune conditions, as well as malignancy. Reported IGD with malignancy includes breast, endometrial, lung, esophageal cancers as well as hematologic malignancies. However, there have not been reports of IGD in patients with prostate cancer. Herein, we report a rare case of IGD associated with prostate cancer, which has not been reported before.