Reports the synthesis of the La2CuO4-related compound HgBa2CuO4+delta (Hg-1201), with only one CuO2 layer per unit cell, and shows that it is superconducting below 94 Kelvin. High-transition-temperature (high-Tc) superconductivity; Crystallographic data for Hg-1201; Observed, calculated and difference powder patterns after Rietveld refinement for Hg-1201; Structure of Hg-1201; Alternating-current magnetic susceptibility measurements; More.