Concurrent PD-1 blockade and thoracic radiotherapy is being investigated in clinical trials for locally advanced, non–small cell lung cancer and small cell lung cancer, despite a potential overlapping risk of cardiotoxicity. Our prior studies demonstrate that cardiotoxicity from concurrent cardiac irradiation and anti–PD-1 administration in a mouse model is CD8+ T-cell dependent. The objective of this study was to determine whether humoral immunity contributed to the observed cardiac tissue damage, as measured by creatine kinase MB and cardiac troponin 1 release and decline in cardiac function. In the current study, we demonstrate the presence of cardiac autoantibodies, which were essential for the occurrence of cardiotoxicity from the combined therapy. Mice subjected to cardiac irradiation, while being treated with anti–PD-1, developed high levels of antibodies that reacted with cardiac tissues in vivo and cardiac antigens in vitro. Moreover, mice deficient in B cells were protected against cardiotoxicity, whereas the transfer of autoantibody-containing sera from mice that had received combined treatment reproduced the same pathologic phenotype in mice exposed to cardiac irradiation but was not observed in normal recipients. The cardiotoxic effect of the sera, which associated with CD8+ T-cell accumulation in cardiac tissue, was limited by IgG depletion. In conclusion, concurrent cardiac irradiation and PD-1 blockade leads to production of cardiac autoantibodies, likely due to antigen exposure within the irradiated cardiac tissues, which play a key role in the resulting cardiotoxicity.