Unilateral breast infiltration caused by T-cell acute lymphoblastic leukemia (T-ALL) is a rare occurrence. This report presents the 18 F-FDG PET/CT findings of unilateral breast relapse in a male patient diagnosed with T-ALL. After systematic therapy, a follow-up PET/CT scan revealed no evidence of FDG-avid lesions. This case highlights the critical role of PET/CT in detecting and characterizing extramedullary disease involvement.