BM mesenchymal stromal cells (BM-MSCs) support multiple myeloma (MM) cell growth, but little is known about the putative mechanisms by which the BM microenvironment plays an oncogenic role in this disease. Cell-cell communication is mediated by exosomes. In this study, we showed that MM BM-MSCs release exosomes that are transferred to MM cells, thereby resulting in modulation of tumor growth in vivo. Exosomal microRNA (miR) content differed between MM and normal BM-MSCs, with a lower content of the tumor suppressor miR-15a. In addition, MM BM-MSC–derived exosomes had higher levels of oncogenic proteins, cytokines, and adhesion molecules compared with exosomes from the cells of origin. Importantly, whereas MM BM-MSC–derived exosomes promoted MM tumor growth, normal BM-MSC exosomes inhibited the growth of MM cells. In summary, these in vitro and in vivo studies demonstrated that exosome transfer from BM-MSCs to clonal plasma cells represents a previously undescribed and unique mechanism that highlights the contribution of BM-MSCs to MM disease progression.