A new polyborate, Sr8MgB18O36, has been synthesized. Its crystal structure was determined from single-crystal X-ray diffraction data, and characterizations were made by differential scanning calorimetry, Fourier transform IR, UV-vis-near-IR diffuse-reflectance, and first-principles calculations. The structure of Sr8MgB18O36 contains a novel isolated anionic group-an 18-membered ring (B18O36)(18-), which is the first found in borates.