Phosphorylated protein (i.e., phosphoprotein) affinity resins and methods for making and using the same are provided. The subject resins include a substrate bonded to aspartate-based tetradentate ligand/metal ion complexes, where the tetradentate ligand/metal ion complexes have high specificity for phosphorylated amino acids. The subject resins find use in a variety of different applications, including phosphoprotein enrichment applications. Also provided are kits and systems that include the subject resins.