Polymer-nanoparticle (NP) composites have attracted renewed attention due to their enhanced mechanical strength combined with potential advantageous electrical, optical or magnetic properties. The interfacial interaction of NPs with the polymer matrix is crucial for the material’s mechanical behavior, however controlling this interface remains a major challenge. Inspired by the adhesion chemistry of mussel fibers, we have investigated a novel approach to incorporate iron oxide nanoparticles (Fe3O4 NPs) into a hydrogel matrix as crosslinkers. A polyethylene glycol polymer terminated by catechol groups is designed to form a hydrogel network crosslinked via coordination bonding at the surfaces of Fe3O4NPs, yielding a stiff and self-healing hydrogel. Due to the reversible nature and unique crosslinking geometry of coordination bonding, the composite hydrogel presents a very different relaxation behavior compared to conventional gel networks. The interfacial coordination interaction can be well-engineered by controlling the inorganic surfaces, polymer ligands and environment pH, thereby determining the material’s relaxation kinetics. In addition, the superparamagnetic property of Fe3O4 NP is preserved after gelation, allowing for design of responsive functional materials. Finally, we have demonstrated that this gelation motif is applicable in various inorganic nanomaterials, which can open up a versatile approach for designing polymer nanocomposite materials with controlled mechanical properties and functionalities.