To improve the understanding of chemo-refractory high-grade serous ovarian cancers (HGSOCs), we characterized the proteogenomic landscape of 242 (refractory and sensitive) HGSOCs, representing one discovery and two validation cohorts across two biospecimen types (formalin-fixed paraffin-embedded and frozen). We identified a 64-protein signature that predicts with high specificity a subset of HGSOCs refractory to initial platinum-based therapy and is validated in two independent patient cohorts. We detected significant association between lack of Ch17 loss of heterozygosity (LOH) and chemo-refractoriness. Based on pathway protein expression, we identified 5 clusters of HGSOC, which validated across two independent patient cohorts and patient-derived xenograft (PDX) models. These clusters may represent different mechanisms of refractoriness and implicate putative therapeutic vulnerabilities. Display omitted •A comprehensive proteogenomic analysis of 242 HGSOC tumors was performed•A lack of Chr17-LOH was observed to be associated with refractoriness•A 64-protein signature predicts refractoriness in multiple tumor cohorts•Pathway-based clustering reveals 5 subtypes validated in independent cohorts Patients with high-grade serous ovarian cancers (HGSOCs) have a poor outcome, with the standard of care not having changed over the decades. A detailed characterization of the proteogenomic landscape of HGSOCs across multiple cohorts and validation studies identifies a distinct signature that predicts with high specificity a subset of patients with chemotherapy-refractory cancers and implicates potential therapeutic vulnerabilities.