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 rf power that metamaterial structures 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.