Acoustic metamaterials offer an approach to reducing the dynamic response of sandwich panels. The key concept underlying this approach is to consider a metamaterial as a highly distributed system of continuous vibration absorbers that introduces multiple stop bands in which the response of the global structure is reduced. Multiple modal frequencies of the absorber system may be tuned to match global resonance frequencies and/or excitation frequencies. Using the assumed modes method, a metamaterial system is designed for integration into the honeycomb core of a representative sandwich panel. The metamaterial system is modeled as an effective distributed complex mass density in the global sandwich-panel model. The cores for two sandwich panels are fabricated using three-dimensional printing technology and then characterized statically and dynamically to determine effective elastic properties and natural frequencies of the cores, as well as loss factors of the vibration absorbers. The sandwich panels, constructed by bonding unidirectional carbon-fiber face sheets to both cores, are tested dynamically for two different boundary conditions: cantilevered and free–free. Experimental results confirm that the metamaterial core reduces the peak dynamic response at the natural frequencies of the sandwich panel with reasonably good agreement with model predictions.