A new mercury-based cuprate superconductor, (Hg 1- x Cr x )Sr 2CuO y (0⩽ x⩽0.8), having T c of 58 K ( x=0.3 has been synthesized. While a chromium-free sample ( x=0) was multi-phased and the superconducting volume fraction was less than 1%, samples for x=0.3 and 0.4 were of nearly single phase and their superconducting shielding volume fraction reached 100% of perfect diamagnetism. X-ray diffraction analysis revealed that its crystal symmetry was tetragonal with lattice parameters a 0=3.840−3.854 Å and c 0=8.639−8.705 Å which slightly varied with x. The observed c 0 value, corresponding to the interlayer distance of superconducting CuO 2 planes, was found to be the shortest among all the known mercury-based “12( n-1) n” superconductors. Compared to the barium-containing compounds, the present barium-free materials have been found to be insensitive to the ambient air atmosphere during the synthesis and also to be quite stable after storage in air.