The purpose of this study is to assess the efficacy and safety of of tafasitamab plus lenalidomide in adults with diffuse large B-cell lymphoma (DLBCL) who have relapsed or are refractory to at least 1 but no more than 3 previous systemic DLBCL treatment regimens and who are not eligible for high-dose chemotherapy (HDC) and autologous stem cell transplantation (ASCT).