A β-D-galactose ligand equipped with a terminal carboxylic acid through a hydrophilic PEG3 spacer. The carboxylic acid can be activated to NHS, sulfo-NHS, or pentafluorophenyl esters and reacted with amine-functionalized platforms or lysine residues on proteins and peptides to form amide bonds. This compound provides a β-anomeric galactose building block for glycoconjugate synthesis, glycomimetic research, and the construction of probes for galactose-recognizing proteins, including galectins and other lectins. Terminal β-Gal residues are also a substrate for glycosyltransferases, including sialyltransferases and fucosyltransferases.