A 72-year-old man presented to the emergency department with a 2-day history of an itchy rash on his back. The intensity of the itching had impaired his ability to sleep. Two days before the onset of the rash, he had prepared and eaten a meal containing shiitake mushrooms. On physical examination, edematous, flagellate plaques and linear patches were present across the patient’s entire back and upper buttocks. There was no adenopathy, dermographism, or mucosal involvement. A diagnosis of shiitake dermatitis was made. Shiitake dermatitis occurs after consumption of raw or undercooked shiitake mushrooms. The characteristic streaky rash — also known as flagellate erythema — may also be seen after exposure to bleomycin or, in rare cases, in dermatomyositis or adult-onset Still’s disease. A dietary history clinches the diagnosis. The patient was reassured about the self-limited nature of the condition and was advised to fully cook shiitake mushrooms in the future. Topical glucocorticoids and oral antihistamines were also given for treatment of the symptoms. At the 2-week follow-up, residual areas of postinflammatory hyperpigmentation were present, but the itchiness had abated.