Autologous hematopoietic stem cell transplantation (ASCT) is a standard-of-care treatment for many hematologic malignancies. Progression of disease after ASCT is the primary cause of treatment failure. In this Phase Ib trial, we studied the safety and clinical effect of combined checkpoint inhibition therapy (CPIT) with ipilimumab and nivolumab as a consolidation strategy after ASCT for patients with high-risk diffuse large B cell lymphoma (DLBCL), mature T cell lymphoma (TCL), and multiple myeloma (MM). Starting at 14 to 28 days after ASCT, patients received ipilimumab (1 mg/kg i.v. on day 1 of weeks 1, 4, 7, 10, 16, and 22) and nivolumab (3 mg/kg i.v. on day 1 of weeks 1, 4, 7, 10, 12, 14, 16, 18, 20, 22, 24, and 26). Patients received a median of 5 doses of ipilimumab and 8 doses of nivolumab. Thirty-five patients were included in the intent-to-treat population. Ninety-four percent of the patients experienced immune-related adverse events (irAEs) of any grade. Ninety-seven percent of irAEs resolved spontaneously or after holding study drugs and instituting high-dose corticosteroid therapy. Progression-free and overall survival at 18 months post-ASCT for each disease cohort were 85.7% and 100% for primary refractory DLBCL, 28.6% and 57.1% for relapsed DLBCL, not evaluable and 80% for frontline TCL, 25% and 75% for relapsed TCL, 57.1% and 87% for high-risk transplant-naïve MM, and 40% and 100% for MM relapsed within 3 years of first ASCT. We conclude that combined CPIT appears to be tolerable as a consolidation strategy after ASCT and in addition to the potential clinical efficacy observed in some subsets of disease, T cell receptor repertoire, T regulatory cell phenotype, and gut microbiota profiles provide a biologic rationale warranting further study of this approach.