A 64-year-old female underwent Bt+Ax surgery due to ER(-), PR(-), HER2(+), ycT3N1M0, Stage ⅢA right breast cancer. After cancer recured in the chest wall, whole-breast radiation therapy was performed, followed by ddAC, and T- DM1. After 12 courses of T-DM1, CT scan and physical findings showed no evidence of metastases, so chemotherapy was suspended with strict follow-up. Seven months later, a chest wall recurrence with pleural dissemination was found and 9 courses of PER plus HER plus eribulin therapy was administered until disease progression. T-DM1 was re-administered but disease progressed after 2 courses accompanied by SVC syndrome due to 8 cm mediastinal lymph node metastasis which caused respiratory discomfort and face edema. We administered T-DXd and after the first course respiratory symptoms vanished, and after 3 courses lymph node metastasis shrunk extremely in the CT imaging. SVCS is one of the oncologic emergencies, in which palliative radiotherapy may be typically selected for the relief of symptoms, and intravascular stents are used in urgent cases. Surprisingly, we experienced a case of SVC syndrome caused by breast cancer metastasis, effectively treated by T-DXd.