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Regulatory T cells (Tregs) are a subset of CD4+ T lymphocytes that suppress the functions of antigen-presenting cells and effector T cells, characterized by the expression of transcription factor forkhead box P3 (FOXP3). Recent studies have reported an increase in the number of Tregs in the bone marrow (BM) of multiple myeloma (MM) patients. However, the role and mechanisms of Treg accumulation in the BM of MM patients remain debatable. Here, we present our data demonstrating the significance of Tregs in the context of MM disease progression. Using the transplantable MM mouse model, we observed a significant increase in Tregs in the BM of MM-injected mice from the early disease stage. We observed extended survival in MM-injected mice with Treg depletion than in mice without Treg depletion, demonstrating direct in vivo evidence that Tregs enhance disease progression in MM. It is noteworthy that type 1 interferon (IFN) signaling is activated in MM-associated Tregs. By using type 1 IFN receptor blocking antibody treatment and type 1 IFN receptor knockout Tregs, we demonstrated a significant decrease in MM-associated Treg proliferation, which was associated with longer survival in MM-injected mice. Thus, we have demonstrated that Tregs play a significant role in MM progression; the function and homeostasis of Tregs are regulated by type 1 IFN secreted in the BM microenvironment.