Objectives Immune checkpoint inhibitors have achieved clinical success in cancer treatment, but this treatment causes immune‐related adverse events, including type 1 diabetes (T1D). Our aim was to test whether a JAK1/JAK2 inhibitor, effective at treating spontaneous autoimmune diabetes in nonobese diabetic (NOD) mice, can prevent diabetes secondary to PD‐L1 blockade. Methods Anti‐PD‐L1 antibody was injected into NOD mice to induce diabetes, and JAK1/JAK2 inhibitor LN3103801 was administered by oral gavage to prevent diabetes. Flow cytometry was used to study T cells and beta cells. Mesothelioma cells were inoculated into BALB/c mice to induce a transplantable tumour model. Results Anti‐PD‐L1‐induced diabetes was associated with increased immune cell infiltration in the islets and upregulated MHC class I on islet cells. Anti‐PD‐L1 administration significantly increased islet T cell proliferation and islet‐specific CD8 + T cell numbers in peripheral lymphoid organs. JAK1/JAK2 inhibitor treatment blocked IFNγ‐mediated MHC class I upregulation on beta cells and T cell proliferation mediated by cytokines that use the common γ chain receptor. As a result, anti‐PD‐L1‐induced diabetes was prevented by JAK1/JAK2 inhibitor administered before or after checkpoint inhibitor therapy. Diabetes was also reversed when the JAK1/JAK2 inhibitor was administered after the onset of anti‐PD‐L1‐induced hyperglycaemia. Furthermore, JAK1/JAK2 inhibitor intervention after checkpoint inhibitors did not reverse or abrogate the antitumour effects in a transplantable tumour model. Conclusion A JAK1/JAK2 inhibitor can prevent and reverse anti‐PD‐L1‐induced diabetes by blocking IFNγ and γc cytokine activities. Our study provides preclinical validation of JAK1/JAK2 inhibitor use in checkpoint inhibitor‐induced diabetes.