Carbohydrates are key mediators of cell-cell, cell-virus, and cell-ligand interactions. These interactions are often difficult to study because they are characterized by fast off rates and low affinities. Photoactivatable crosslinking sugars can be used to capture these transient interactions by forming a covalent bond between the carbohydrate and its binding partner. We designed and synthesized 'photo-sugars' that contain diazirine moieties on the N-acyl side chains. We demonstrated that these photo-sugars could be metabolically incorporated onto the surface of living cells. Upon UV irradiation, specific crosslinks were formed between glycoprotein binding partners. Importantly, photo-sugar incorporation competes efficiently with endogenous carbohydrate biosynthesis and is compatible with the sialic acid modifications, such as 9-O-acetylation, that regulate binding events.