Post-translational modifications (PTMs) regulate numerous proteins and are important for many biological processes. Lysine 4-oxononanoylation (4-ONylation) is a newly discovered histone PTM that prevents nucleosome assembly under oxidative stress. Whether there are cellular enzymes that remove 4-ONyl from histones remains unknown, which hampers the further investigation of the cellular function of this PTM. Here, we report that mammalian SIRT2 can remove 4-ONyl from histones and other proteins in live cells. A crystal structure of SIRT2 in complex with a 4-ONyl peptide reveals a lone pair-pi interaction between Phe119 and the ketone oxygen of the 4-ONyl group. This is the first time that a mechanism to reverse 4-ONyl lysine modification is reported and will help to understand the role of SIRT2 in oxidative stress responses and the function of 4-ONylation.