Aberrant alternative splicing is one of the hallmarks of cancer and is potentially based on upregulated expression-of-splicing factors in some types of cancer. Our previous study suggested that the splicing factor RBM39 is significantly upregulated in multiple myeloma (MM) and that its upregulation is positively associated with poor prognosis. Here, we further demonstrate that the survival and proliferation of MM cells rely on RBM39 and that RBM39 knockdown inhibits the malignant growth of MM. Indisulam, a "molecular glue" that mediates the proteasomal degradation of RBM39, has potent suppressive effects on MM both in vitro and in vivo. Deletion of RBM39 results in extensively altered splicing, with mis-splicing of MEK5 verified to inhibit the malignant growth of MM. Full-length MEK5 plays a vital role in maintaining MM cell survival, whereas aberrant MEK5 isoforms with exon loss exhibit loss of function and a propensity for proteasomal degradation. Targeting RBM39 or MEK5 synergistically increases the cytotoxicity of bortezomib in MM cells via the inhibition of p65. Our study validates the specific mechanism of RBM39 in MM, providing an approach for broader targeting and optimized therapeutic strategies for MM.© 2025 American Society of Hematology. Published by Elsevier Inc. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.