A 77-year-old transgender man (assigned female sex at birth, gender identity male, i.e. female-to-male) was referred for a palpable mass of the right chest wall. Biopsies revealed invasive lobular breast carcinoma. After discussion by a multidisciplinary tumour board meeting, the patient was treated with total mastectomy, adjuvant hypofractionated radiation therapy, and hormone therapy. At 1.5-year follow-up, there was no sign of recurrence or long-term radiation side effects. To our knowledge, this is the first reported case of adjuvant hypofractionated radiation therapy in a transgender patient with breast cancer.