Updated project metadata. Multiple myeloma is a plasma cell malignancy of the bone marrow. Despite therapeutic advances, multiple myeloma remains incurable and better risk stratification as well as new therapies are therefore highly needed. The proteome of multiple myeloma has not been systematically assessed before and holds the potential to uncover additional insight into disease biology and improved prognostic models. Here, we provide a comprehensive multi-omics analysis including deep tandem mass tags (TMT)-based quantitative global (phospho)proteomics, RNA sequencing and nanopore DNA sequencing of 138 primary patient-derived plasma cell malignancies encompassing treatment-naive multiple myeloma patients treated in clinical trials, plasma cell leukemia, and the premalignancy monoclonal gammopathy of undetermined significance (MGUS), as well as healthy controls. We found that the (phospho)proteome of malignant plasma cells is highly deregulated as compared to healthy plasma cells and is both defined by chromosomal alterations and extensive post-transcriptional regulation. A protein signature was identified that is associated with aggressive disease and more predictive for outcome than cytogenetic-based risk assessment in newly diagnosed multiple myeloma. Integration with functional genetics and single-cell sequencing revealed generally and genetic subtype-specific deregulated proteins and pathways in plasma cell malignancies that include novel potential targets for (immuno)therapies. These findings provide new insights in the biology of multiple myeloma and will be a unique resource for investigating new therapeutic approaches.