We identify the extended Einstein IPC X-ray source 1E 0657-56 with a previously unknown cluster of galaxies at a redshift of z=0.296. Optical CCD images show the presence of a gravitational arc in this cluster, and galaxy spectra yield a cluster velocity dispersion of 1213+ 352−191 km s−1. X-ray data obtained with the ROSAT HRI and ASCA indicate that 1E 0657-56 is a highly luminous cluster in which a merger of subclusters may be occurring. The temperature of the hot gas in 1E 0657-56 is kT=17.4±2.5 keV, which makes it an unusually hot cluster, with important cosmological implications.