Methyltetrazine-amido-N-bis(PEG4-acid) is a click chemistry reagent featuring a tetrazine group, capable of undergoing inverse electron demand Diels-Alder (iEDDA) reactions with molecules containing TCO groups. This PEGylated derivative possesses a methyltetrazine moiety and two acidic groups. It forms stable covalent bonds with TCO-bearing compounds through a rapid and selective Diels-Alder cycloaddition, without the need for Cu catalysis or high temperatures. Additionally, its terminal carboxylic acids can react with primary amines in the presence of activators (such as EDC or HATU) to form stable amide bonds. The PEG linkage enhances the compounds water solubility. This reagent is for research use only.