A thiofunctional thiazolidine is introduced as a new low-molar-mass building block for the introduction of cysteine residues via a thiol-ene reaction. Allyl-functional polyglycidol (PG) is used as a model polymer to demonstrate polymer-analogue functionalization through reaction with the unsaturated side-chains. A modified trinitrobenzenesulfonic acid (TNBSA) assay is used for the redox-insensitive quantification and a precise final cysteine content can be predetermined at the polymerization stage. Native chemical ligation at cysteine-functional PG is performed as a model reaction for a chemoselective peptide modification of this polymer. The three-step synthesis of the thiofunctional thiazolidine reactant, together with the standard thiol-ene coupling and the robust quantification assay, broadens the toolbox for thiol-ene chemistry and offers a generic and straightforward approach to cysteine-functional materials.