Poly(l-cysteine)s with methylthio pendants (PMTLCs) were synthesized by ring-opening polymerization of a new l-cysteine-based N-carboxyanhydride. The thioether bonds of PMTLC can be readily oxidized by H2O2 yielding water-soluble PMTLCOX. The methylthio groups can undergo an alkylation reaction using methyl iodide and a subsequent ion-exchange reaction yielding sulfonium-based polypeptides (PPLC-DMS-X, where X = I, BF4). PPLC-DMS-X showed upper critical solution temperature-type thermo- and oxidation-responsive properties in aqueous solutions. Both PMTLC and PPLC-DMS-X showed oxidation-induced β-sheet to α-helix transitions. The absorbance of PPLC-DMS-I and methyl orange aqueous solution displayed a significant linear correlation with temperature, which makes the sulfonium-based polypeptides good candidates in the field of temperature sensors.