A new class of glycine derivative-functionalized metal-organic framework (MIL-101-glycine, MIL-101-diglycine, MIL-101-triglycine) was successfully prepared by a post-synthesis method and used to remove cobalt ions from an aqueous solution. The results indicated that the Co(II) adsorption was consistent with the Langmuir model, and the maximum saturation adsorption capacity reached 232.6 mg/g for MIL-101-triglycine, which is a higher value of cobalt ion adsorbent obtained to date. The thermodynamic and kinetic parameters showed that the adsorption process is spontaneous and endothermic and proceeds by pseudo-second-order chemisorption, while the adsorption mechanism is dominated by a coordination effect between the cobalt ions and the ligands of the adsorbent, as elucidated by X-ray photoelectron spectroscopy (XPS). In particular, the effect of the ligands on the adsorption behaviour of the composite materials for cobalt ions was evaluated, and it was observed that the adsorption capacity increased with the length of the ligand chain of the glycine derivatives. All findings of this work suggested that MIL-101 is a potential candidate material for the separation of cobalt ions and that an appropriate length of the ligand chain for MOF modification should be chosen.