The thiol (-SH) of the active cysteine residue in peroxiredoxin (Prx) is known to be reversibly hyperoxidized to cysteine sulfinic acid (-SO 2H), which can be reduced back to thiol by sulfiredoxin/sestrin. However, hyperoxidized Prx of an irreversible nature has not been reported yet. Using an antibody developed against the sulfonylated (-SO3H) yeast Prx (Tsa1p) active-site peptide (AFTFVCPTEI), we observed an increase in the immunoblot intensity in proportion to the H2O2 concentrations administered to the yeast cells. We identified two species of hyperoxidized Tsa1p: one can be reduced back (reversible) with sulfiredoxin, and the other cannot (irreversible). Irreversibly hyperoxidized Tsa1p was identified as containing the active-site cysteine sulfonic acid (Tsa1p-SO 3H) by mass spectrometry. Tsa1p-SO3H was not an autoxidation product of Tsa1p-SO2H and was maintained in yeast cells even after two doubling cycles. Tsa1p-SO3H self-assembled into a ring-shaped multimeric form was shown by electron microscopy. Although the Tsa1p-SO3H multimer lost its peroxidase activity, it gained ∼4-fold higher chaperone activity compared with Tsa1p-SH. In this study, we identify an irreversibly hyperoxidized Prx, Tsa1p-SO3H, with enhanced molecular chaperone activity and suggest that Tsa1p-SO3H is a marker of cumulative oxidative stress in cells.