In this work, an amino-modified cellulose nanofiber sponge was prepared and used as a support for polyoxometalate (POM) catalysts with a high loading efficiency. Fourier transform infrared spectroscopy, thermogravimetric analysis, and energy-dispersive X-ray spectroscopy revealed that an Anderson-type POM, (NH₄)₄[CuMo₆O₁₈(OH)₆]·5H₂O was successfully immobilized on the sponge based on electrostatic interactions. Morphological analysis indicated that the POM-loaded sponge retained its porous structure and that the POM was homogeneously distributed on the sponge walls. The POM-loaded sponge exhibited excellent mechanical properties by recovering 79.9% of its original thickness following a 60% compression strain. The POM-loaded sponge was found to effectively catalyze the hydroboration of phenylacetylenes, yielding excellent conversion and regioselectivity of up to 96% and 99%, respectively. Its catalytic activity remained unchanged after five reuse cycles. These findings represent a scalable strategy for immobilizing POMs on porous supports.