N-(Azido-PEG2)-N-Boc-PEG3-NHS ester is a branched PEG derivative with a terminal azide group, Boc protected amino group, and a terminal NHS ester. The azide group enables PEGylation via Click Chemistry. The protected amine can be deprotected by acidic conditions. The NHS ester can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules.