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We report on our serendipitous discovery with the XMM-Newton Observatory of a luminous X-ray emitting cluster of galaxies that is located behind the Andromeda galaxy (M31). X-ray emission from the cluster was detected previously by ROSAT, and cataloged as RX J0046.4+4204, but it was not recognized as a galaxy cluster. The much greater sensitivity of our XMM-Newton observations revealed diffuse x-ray emission that extends at least 5 arcmin and has a surface brightness profile that is well fit by the alpha-beta model with beta = 0.70 +/- 0.08, a core radius r_c = 56 arcsec +/- 16, and alpha = 1.54 +/- 0.25. A joint global spectral fit of the EPIC/MOS1, MOS2, and PN observations with Mewe-Kaastra-Liedahl plasma emission model gives a cluster temperature of 5.5 +/- 0.5 keV. The observed spectra also show high significance iron emission lines that yield a measured cluster redshift of z = 0.290 with a 2% accuracy. For a cosmological model with H_0 = 71 km s^{-1} Mpc^{-1}, Omega_M = 0.3 and Omega_{Lambda} = 0.7 we derive a bolometric luminosity of L_x=(8.4 +/- 0.5)*10^{44}$ erg/s. This discovery of a cluster behind M31 demonstrates the utility of x-ray surveys for finding rich clusters of galaxies, even in directions of heavy optical extinction.