Humoral immunity plays a substantial role in the suppression of breast cancer. We have revealed that a high serum concentration of anti-HER2 autoantibody (HER2-AAb) is associated with favorable outcomes in patients with invasive breast cancer. Thus, we aimed to clarify the association between high serum concentration of HER2-AAb and humoral immune response in the tumor microenvironment. Out of 500 consecutive patients with invasive breast cancer, we selected those whose HER2-AAb values were high (n = 33) or low (n = 20) based on the distribution of HER2-AAb values of 100 healthy individuals. Tumor and regional lymph node formalin-fixed paraffin-embedded samples prepared from the surgical specimens were subjected to immunohistochemistry. We confirmed that the recurrence-free survival of the high HER2-AAb group was significantly longer than that of the low HER2-AAb group (p = 0.015). The numbers of tumor-infiltrating CD20+ immune cells (ICs) (p < 0.001), IGKC+ICs (p = 0.023), and CXCL13+ ICs (p = 0.044) were significantly higher in the high HER2-AAb group than in the low HER2-AAb group. The number of CD4+ ICs in the B-cell follicles of the regional lymph nodes was also significantly greater in the high HER2-AAb group than in the low HER2-AAb group (p = 0.026). Our findings indicate that a high level of HER2-AAb is associated with enhanced humoral immunity against breast cancer and thus may provide a rationale for the association of HER2-AAb with favorable prognosis.