When proteostasis becomes unbalanced, unfolded proteins accumulate and can aggregate. However, probes quantifying proteostasis imbalance are lacking. We report a dye, tetraphenylethene maleimide (TPE-MI) that can measure unfolded protein load.TPE-MI fluorescence is enhanced upon reaction with cellular proteomes under conditions promoting accumulation of unfolded proteins and, via proteomic analysis, can track which proteins expose more cysteine residues under stress.