Chemical reporters are unique functional groups that can be used to label biomolecules in living systems. Only a handful of broadly applicable reporters have been identified to date, owing to the rigorous demands placed on these functional groups in biological settings. We describe here a new chemical reporter-cyclopropene-that can be used to target biomolecules in vitro and in live cells. A variety of substituted cyclopropene scaffolds were synthesized and found to be stable in aqueous solution and in the presence of biological nucleophiles. Furthermore, some of the cyclopropene units were metabolically introduced into cell surface glycans and subsequently detected with covalent probes. The small size and selective reactivity of cyclopropenes will facilitate efforts to tag diverse collections of biomolecules in vivo.