We report the discovery of a protocluster at z ∼ 6 containing at least eight protocluster members with spectroscopic confirmations in the wide-field image of the Subaru Deep Field. The overdensity of the protocluster is significant at the 6σ level, based on the surface number density of i′-dropout galaxies. The overdense region covers ∼ 6′ × 6′ and includes 30 i′-dropout galaxies. Follow-up spectroscopy revealed that 15 of these are real z ∼ 6 galaxies. Eight of the 15 are clustering in a narrow redshift range (Δz < 0.05), corresponding to a seven-fold increase in number density over the average in redshift space. We found no significant difference in the observed properties between the eight members and the seven non-members. The velocity dispersion of the eight members is 647 ± 124kms−1, which is about three times higher than that predicted by the standard cold dark matter model. This discrepancy could be attributed to the distinguishing three-dimensional distribution of the eight members. We discuss two possible explanations for this discrepancy: either the protocluster is already mature, with old galaxies at the center, or it is still immature and composed of three subgroups merging to become a larger cluster. In either case, this concentration of z ∼ 6 galaxies may be one of the first sites of formation of a galaxy cluster.