T cell-mediated destruction of insulin-producing beta cells in the pancreas causes type 1 diabetes T1D). CD4 T cell responses play a central role in beta cell destruction, but the identity of the epitopes recognized by pathogenic CD4 T cells remains unknown. We found that diabetes-inducing CD4 T cell clones isolated from nonobese diabetic mice recognize epitopes formed by covalent cross-linking of proinsulin peptides to other peptides present in beta cell secretory granules. These hybrid insulin peptides HIPs) are antigenic for CD4 T cells and can be detected by mass spectrometry in beta cells. CD4 T cells from the residual pancreatic islets of two organ donors who had T1D also recognize HIPs. Autoreactive T cells targeting hybrid peptides may explain how immune tolerance is broken in T1D.