The HER2/ErbB2 monoclonal antibody (mAb) trastuzumab is combined with chemotherapy as a standard-of-care for newly diagnosed HER2⁺ breast cancer patients, but some patients treated with this combination therapy experience early relapse. Our analysis of data from a clinical trial evaluating the efficacy of chemotherapy plus/minus trastuzumab suggested that the magnitude of trastuzumab benefit on distant disease-free survival was higher for increasing expression of the IL21 receptor (IL21R). Therefore, we investigated a possible role for IL21 signaling in promoting HER2 mAb therapeutic efficacy. We found that IL21R-deficient mice and wild-type mice treated with a neutralizing anti-IL21 mAb were less susceptible to trastuzumab-like anti-ErbB2 therapy. Furthermore, IL21R expression on CD8⁺ T cells, but not on natural killer cells, was required for optimal anti-ErbB2 mAb efficacy, and IL21 expression was enhanced in tumor-infiltrating CD4⁺ T lymphocytes after anti-ErbB2 therapy. Finally, we found that administering recombinant IL21 in combination with anti-ErbB2 therapy was therapeutic against primary tumors and experimental metastases in mice. Collectively, our findings suggest that elevating IL21 signaling may enhance trastuzumab efficacy, thus constituting a novel candidate strategy to overcome trastuzumab resistance and improve patient survival. Cancer Res; 76(2); 264–74. ©2016 AACR.