Holmium (Ho), one of the lanthanide elements, shows a high magnetic moment. Here we present a simple, yet highly potential approach for preparing polymer-based magnetic materials from a three-dimensional polymer network composed with poly(acrylic acid) and Ho showing trivalent nature. We have successfully prepared a magnetic polymer network that reacts directly to a magnet by three-dimensionally immobilizing Ho in the polymer matrix. The present method allowed a preparation of wide range of magnetic materials using polymeric scaffolds, e.g., polymer-grafted particles and cross-linked polymer gels. As a result of the high Ho content, these materials responded quickly to the magnet. The discovery of a method to prepare magnetic materials will provide flexibility in materials design and greatly expand the scope of application of magnetic materials.