We report the case of a 44-year-old male who underwent mastectomy plus axillary lymph node dissection after neoadjuvant chemotherapy for left breast cancer. During adjuvant therapy, multiple bone metastases and pericardial effusion were detected. Pericardial drainage was mandated for the patient, although no malignant cells were found. Despite the change in treatment, pericardial effusion worsened. Based on clinical findings, the patient was diagnosed with carcinomatous pericarditis and was switched to bevacizumab plus paclitaxel therapy. CT at 5 months showed improved pericardial effusion; treatment is ongoing.