Galaxy clusters trace the largest structures of the Universe and provide ideal laboratories for studying galaxy evolution and cosmology1,2. Clusters with extended X-ray emission have been discovered at redshifts of up to z ≈ 2.5 (refs 3,4,5,6,7). Meanwhile, there has been growing interest in hunting for protoclusters, the progenitors of clusters, at higher redshifts8,9,10,11,12,13,14. It is, however, very challenging to find the largest protoclusters at early times, when they start to assemble. Here, we report a giant protocluster of galaxies at z ≈ 5.7, when the Universe was only one billion years old. This protocluster occupies a volume of about 353 cubic comoving megaparsecs. It is embedded in an even larger overdense region with at least 41 spectroscopically confirmed, luminous Lyα-emitting galaxies (Lyα emitters, or LAEs), including several previously reported LAEs9. Its LAE density is 6.6 times the average density at z ≈ 5.7. It is the only one of its kind in an LAE survey in 4 deg2 on the sky. Such a large structure is also rarely seen in current cosmological simulations. This protocluster will collapse into a galaxy cluster with a mass of (3.6 ± 0.9) × 1015 solar masses, comparable to those of the most massive clusters or protoclusters known so far.