Multiple myeloma (MM) is the second most prevalent hematological malignancy and remains incurable with remarkable heterogeneity in prognosis and treatment response across the patients. Clinical diagnosis and the existing molecular classification systems are inadequate for predicting treatment responses. Based on the convergence between plasma cell development and MM pathogenesis, we identified a gene co-expression module centered on the plasma cell survival regulator MCL1 (MCL1 module, MCL1-M) in the transcriptomes of pre-treated MM, which enabled stratification of MM patients into MCL1-M high and MCL1-M low molecular subtypes with subtype-specific prognosis and response to bortezomib-containing treatment. Here, we aimed to examine the mechanism underlying the disparate prognosis and treatment responses between the two molecular subtypes. Our findings reveal that MCL1-M high MM displays significant activation of pathways associated with cell proliferation, while MCL1-M low MM exhibits activation of immune-related signaling pathways. The relative enrichment of immune cells within the bone marrow microenvironment of MCL1-M low MM, particularly plasmacytoid dendritic cells, likely contributes to the activation of immune-related signaling pathways in this subset of myeloma cells. Using phase III trial data, we show that responses to bortezomib-containing treatment are associated with the extent of unfolded protein response (UPR) signaling activity. Further, bortezomib-mediated killing of MM cells could be enhanced or inhibited by in vitro manipulation of UPR activities in representative cell lines. In conclusion, MCL1-M based molecular subtypes of MM are characterized by distinct signaling activities from both malignant cells and bone marrow microenvironment, which may drive distinct prognosis and treatment responses.