THE recession velocities of relatively nearby galaxies show systematic deviations from a uniform expansion field, These deviations indicate the presence of the ’Great Attractor’-a large concentration of mass (similar to 5 x 10(16) solar masses) that lies in the direction of the southern Milky Way(1,2), Attempts to quantify both the nature and extent of the Great Attractor(3-11) have been hampered by the fact that it is largely hidden by the disk of the Milky Way. Although there is an excess of galaxies in this region(12), no dominant galaxy cluster or other concentration of mass has hitherto been identified, Here we present results from a survey of galaxies obscured (or partially obscured) by the southern Milky Way, Our results show that a previously identified galaxy cluster, Abell 3627, which lies only 9 degrees from the predicted centre of the Great Attractor(13), is very massive (similar to 5 x 10(15) solar masses). The cluster’s redshift is also near that predicted for the core of the Great Attractor(2,3,12-14), suggesting that it lies at or near the bottom of the Attractor’s gravitational potential well.