Primary mediastinal large B-cell lymphoma (PMBCL) is a rare hematologic malignancy with distinct clinical and immunopathological features. We report a case of a young adult male with disease refractory to multiple lines of therapy, including CAR-T cells, who achieved his first complete remission after haploidentical BMT, following donor leukocyte infusions (DLI) given concurrently with blinatumomab. While DLI has been used after T-replete haplo-BMT with PT-CY, there are no reports on its use for PMBCL. Similarly, blinatumomab is active against B-cell lymphomas, but literature is lacking in patients with PMBCL. Our experience illustrates that blinatumomab can be used concurrently with DLI in a haploidentical setting to achieve disease response in PMBCL. Despite our encouraging experience with this case, we would not recommend this approach outside of a clinical trial as blinatumomab may exacerbate the GvHD risks of DLI especially in a haploidentical setting.