Outcomes and prognostic factors of reduced intensity-conditioned allo-SCT (RIC allo-SCT) for multiple myeloma (MM) relapsing or progressing after prior autologous (auto)-SCT are not well defined. We performed an analysis of 413 MM patients who received a related or unrelated RIC allo-SCT for the treatment of relapse/progression after prior auto-SCT. Median age at RIC allo-SCT was 54.1 years, and 44.6% of patients had undergone two or more prior auto-SCTs. Median OS and PFS from the time of RIC allo-SCT for the entire population were 24.7 and 9.6 months, respectively. Cumulative non-relapse mortality (NRM) at 1 year was 21.5%. In multivariate analysis, CMV seronegativity of both patient and donor was associated with significantly better PFS, OS and NRM. Patient–donor gender mismatch was associated with better PFS, fewer than two prior auto-SCT was associated with better OS, and shorter time from the first auto-SCT to the RIC allo-SCT was associated with lower NRM. The results of this study identify patient and donor CMV seronegativity as the key prognostic factor for outcome after RIC allo-SCT for MM relapsing or progressing after prior auto-SCT.