BACKGROUND Infiltrating ductal breast carcinoma with monoclonal gammopathy of undetermined significance (MGUS) is rare and easily misdiagnosed. Most patients are first diagnosed with MGUS. We report a rare case of MGUS secondary to infiltrating ductal breast carcinoma. We also review the literature to analyze the clinical characteristics and diagnostic methods. CASE SUMMARY A 51-year-old woman underwent modified radical mastectomy for infiltrating ductal carcinoma of the right breast and was then treated with radiation and chemotherapy. A decreased platelet count was found on routine blood examination, and MGUS was subsequently diagnosed. This is the first report of the occurrence of MGUS after breast cancer surgery. CONCLUSION Vigilance is required to distinguish this rare comorbidity from breast plasmacytoma.