Darier disease (DD) is a rare autosomal dominant keratinizing disorder often characterized by brown scaly pruritic papules over the face, neck, and trunk. Herein is reported a patient who developed secondary cutaneous herpes simplex virus (HSV) following exacerbation of his DD as a result of radiation therapy. In November 2020, a 78-year-old man presented to clinic for a pruritic rash on his back consistent with DD. He had developed the rash after the conclusion of chemoradiation therapy for recently diagnosed urothelial carcinoma of the bladder with squamous differentiation. However, he returned two weeks later complaining of a marked worsening of the rash associated with a pain and burning sensations. Histopathology was non-conclusive, but the lesions were found to be positive for HSV-1 by PCR. The patient recovered without complication over a period of two weeks following a course of valacyclovir. There is precedent in the literature for ionizing radiation inducing flares of DD lesions in overlying skin. In addition, DD has been shown to put a patient at increased risk for secondary infections such as HSV. This case report demonstrates that HSV could pose a significant risk to those with DD receiving radiation therapy and thus could warrant prophylactic treatment.