Checkpoint inhibitors effectively enhance the natural immune response against cancer, but they are also known to induce a unique spectrum of immune-related adverse events. Here, we report the first case of isolated neutropenia subsequent to nivolumab therapy. Prominent activated T-cells were found in the patient's serum and bone marrow alongside evidence of maturational defects in neutrophil precursors. Antineutrophil antibodies were not detected despite reliable testing techniques. A T-cell-mediated response is probable, consistent with the established mechanism for the development of other immune-related toxicities. Awareness of this rare and severe side effect reinforces the importance of early diagnosis and prompt initiation of proper treatment.