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We report the detection of a massive high-redshift cluster of galaxies nearthe quadruple quasar RX J0911+05, using the LRIS instrument on the Keck-IItelescope. The cluster is found to have a mean redshift of <z>=0.7689+/-0.002and a velocity dispersion of sigma=836{+180-200} km/s, based on redshiftmeasurements for 24 member galaxies. This massive high-redshift cluster is theorigin of the unusually large external shear required by lensing models of thequadruple quasar system. We predict the expected time delay depending on theexact contribution of the cluster. A measurement of the time delay and furtherdeep lensing and X-ray observations will unravel useful properties of thisserendipitously discovered high-redshift cluster, and may put interestingcosmological constraints on H0.Comment: Submitted to ApJL, 7 pages, 5 figure

RX J0911+05: A Massive Cluster Lens at [ITAL]z[/ITAL] = 0.769