Updated project metadata. Nitro-fatty acids (NFAs) are endogenously formed lipid mediators that cause a broad spectrum of anti-inflammatory effects by covalent modification of key proteins within inflammatory signaling pathways. Their potential as therapeutics has been demonstrated in numerous animal models of disease. Herein, we evaluated the anti-tumorigenic effects of NFAs in the colon carcinoma cell line Caco-2 and other solid and leukemic tumor cell lines. NFAs inhibited the ubiquitin–proteasome system (UPS), leading to polyubiquitination and inhibition of the proteasome activities. Suppression of the UPS induced the pro-apoptotic transcription factor CHOP by activation of the unfolded protein response (UPR), resulting in tumor cell death. NFA-mediated effects on UPS and UPR were rather strong, highly specific, largely irreversible, and represent a mechanistically new mode of action for pharmacological suppression of the UPS. Taking into account their favorable safety profile in clinical phase I trials in combination with their well-recognized anti-tumorigenic efficacy covering solid tumors, we conclude that NFAs are innovative proteasome inhibitor drug candidates that may have a large potential to overcome the limitations of the classical proteasome inhibitors.