Although reactive polytypic large granular lymphocyte (LGL) expansions after hematopoietic stem cell transplant (HSCT) due to viral infection are common, T-cell large granular lymphocytic leukemia (T-LGLL) is extremely rare and usually clinically indolent and requires no treatment in the post-HSCT setting. In contrast, here we present a case of post-HSCT donor-derived T-LGLL that led to pure red cell aplasia with severe patient morbidity. After diagnosis, the patient received immunosuppressive treatment with a dramatic response and has been transfusion independent for more than a year. To our knowledge, this is the first case of a post-HSCT T-LGLL with a documented significant adverse clinical impact requiring and responding to therapeutic intervention. It underscores the importance of recognizing such a rare entity in posttransplant patients.