The biological function of interleukin-10 (IL-10), a pleiotropic cytokine with an essential role in inflammatory processes, is known to be affected by glycosaminoglycans (GAGs). GAGs are highly negatively charged polysaccharides and integral components of the extracellular matrix with important functions in the biology of many growth factors and cytokines. The molecular mechanism of the IL-10-GAG interaction is unclear. In particular, experimental evidence about IL-10-GAG binding sites is lacking, despite its importance for understanding the biological role of the interaction. Here, we report the experimental determination of a GAG binding site of IL-10. Whereas no co-crystal structure of the IL-10-GAG complex could be obtained, its structural characterization was possible by NMR spectroscopy. Chemical shift perturbations of IL-10 induced by GAG binding were used to narrow down the location of the binding site and to assess the affinity for different GAG molecules. Subsequent observation of NMR pseudocontact shifts (PCSs) of IL-10 and its heparin ligand as induced by a protein-attached lanthanide spin label provided structural restraints for the protein-ligand complex. Using these restraints, PCS-based rigid body docking together with molecular dynamics simulations yielded a GAG binding model. The heparin binding site is located at the C-terminal end of helix D and the adjacent DE loop and coincides with a patch of positively charged residues involving arginines R102, R104, R106, R107 and lysines K117 and K119. This study represents the first experimental characterization of the IL-10-GAG complex structure and provides the starting point for revealing the biological significance of IL-10's interaction with GAGs.