Lysine methylation is an important histone post translational modification (PTM) for manipulating chromatin structure and regulating gene expression, and its dysregulation is associated with various diseases including many cancers. While characterization of Lys methylation has seen improvements :over the past decade due to advances in proteomic mass spectrometry and methods involving antibodies, chemical methods for selective detection of proteins containing PTMs are still lacking. Here, we detail the,development of a unique labeling method wherein a synthetic receptor probe for trimethyl lysine (Kme3), CX4-ONBD, is used to direct selective fluorescent labeling of Kme3 histone peptides. This supramolecular approach reverses the paradigm of ligand-directed affinity labeling by making the receptor the synthetic component and the ligand the Component to be labeled. We show that the probe mediates a strong turn on fluorescence response in the presence of a Kme3 histone peptide and shows > 5-fold selectivity in covalent labeling over an unmethylated lysine peptide. We also demonstrate the utility of the probe through the design of a turn-on fluorescence assay for histone deacetylase (HDAC) activity and for inhibitor screening and IC50 detertnination. Our synthetic receptor-mediated affinity labeling approach broadens the scope of PTM detection by,chemical means and may facilitate the development of more versatile in vitro enzymatic assays.