Secondary central nervous system (CNS) lymphoma is a rare and often fatal complication of non-Hodgkin lymphoma (NHL). Treatment options include radiation therapy, high-dose systemic chemotherapy, intrathecal chemotherapy, and high-dose chemotherapy with autologous stem cell rescue, but outcomes remain poor. Allogeneic blood or marrow transplant (alloBMT) is widely used in relapsed/refractory systemic NHL. We sought to understand whether a graft-versus-lymphoma effect could maintain remission in CNS disease. Here we review outcomes in 20 consecutive patients with secondary CNS lymphoma who underwent alloBMT with non-myeloablative conditioning using fludarabine, cyclophosphamide, and 200cGy total-body irradiation. For graft-versus-host disease (GVHD) prophylaxis, all patients received post-transplant cyclophosphamide (PTCy), mycophenolate mofetil, and a calcineurin inhibitor. With a median follow up of 4.1 years, the median overall survival for the entire cohort was not reached. Median progression-free survival was 3.8 years (95% confidence interval [CI] 5.3 months – not reached). The cumulative incidence of relapse was 25% (95% CI 5-45%), and non-relapse mortality was 30% (95% CI 5-54%) at 4 years. Of the 5 patients who relapsed, 2 were CNS only, 1 was systemic only, and 2 were combined CNS / systemic. The use of alloBMT in CNS lymphoma deserves further investigation.