Despite substantial advancements in psoriatic arthritis (PsA) treatment modalities, a considerable proportion of patients continue to experience persistent joint inflammation, unresponsive to the armamentarium of disease-modifying antirheumatic drugs. Identifying previously unknown biomarkers and targets for refractory disease is urgently needed. Here, using single-cell RNA sequencing of synovial fluid from 41 patients with PsA and 9 osteoarthritis (OA) controls, we mapped the immune landscape of the pathogenic synovial fluid in patients with PsA. Our analysis revealed profound alterations in the myeloid compartment, primarily in type 2 conventional dendritic cells and monocytes. A comparison of these myeloid programs in PsA patient groups identified elevated expression of genes associated with the immunoproteasome and major histocompatibility complex class I as a major perturbation in refractory patients. Functional in vivo and in vitro experiments using a selective immunoproteasome inhibitor attenuated the activated myeloid compartment and disease manifestations. Our research imparts critical insights into PsA pathogenesis, potentially paving a way for targeted therapeutic interventions in treatment-resistant patients.