In breast cancer surgery, establishing a diagnosis other than lymph node metastasis of breast cancer, when performing a sentinel node biopsy in individuals with breast cancer, is rare. Here, we report a case of incidental discovery of follicular lymphoma by sentinel lymph node biopsy for Paget's disease associated with invasive breast cancer. A 60-year-old female initially presented with erosion on her left nipple and was clinically diagnosed with stage IA human epidermal growth factor receptor type 2 (HER2)-positive breast cancer and Paget's disease. Accordingly, skin-sparing mastectomy, sentinel lymph node biopsy, and immediate breast reconstruction were performed. Although an intraoperative pathological examination of frozen sections of lymph nodes presented no evidence of metastasis, it revealed large follicles. Based on immunohistochemistry of the additional lymph nodes, she was diagnosed with follicular lymphoma. Therefore, we initiated chemotherapy for follicular lymphoma followed by trastuzumab. At present, 6 years after the operation for breast cancer, the patient is doing well.