Abstract: In this talk, the experimental demonstration of a metamaterial-based metallic structure for wakefield acceleration will be presented. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation. At the Argonne Wakefield Accelerator, 65-MeV electron bunches were sent through the structure to excite high-power reversed-Cherenkov wakefield radiation at 11.4 GHz. Single 45 nC electron bunches traversing the structure generated up to 25 MW in 2-ns pulses, in good agreement with theory. Two bunches of 85 nC in total with appropriate temporal spacing generated up to 80 MW by coherent wakefield superposition, the highest radio-frequency power that metamaterial structures have ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high power generation and high-gradient acceleration, the simple and rugged structure, and a large parameter space for optimization.