Massive clusters of galaxies have been found that date from as early as 3.9 billion years1 (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs2, 3. Cosmological simulations using the current cold dark matter model predict that these systems should descend from ‘protoclusters’—early overdensities of massive galaxies that merge hierarchically to form a cluster4, 5. These protocluster regions themselves are built up hierarchically and so are expected to contain extremely massive galaxies that can be observed as luminous quasars and starbursts4, 5, 6. Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe6, 7. Here we report a protocluster region that dates from 1 Gyr (z = 5.3) after the Big Bang. This cluster of massive galaxies extends over more than 13 megaparsecs and contains a luminous quasar as well as a system rich in molecular gas8. These massive galaxies place a lower limit of more than 4 × 1011 solar masses of dark and luminous matter in this region, consistent with that expected from cosmological simulations for the earliest galaxy clusters4, 5, 7.