Aims: Superclusters are the largest relatively isolated systems in the cosmic web. Using the SDSS BOSS survey, we search for the largest superclusters in the redshift range 0.43 < z < 0.71. Methods: We generate a luminosity-density field smoothed over 8 h-1Mpc to detect the large-scale over-density regions. Each individual over-density region is defined as single supercluster in the survey. We define the superclusters so that they are comparable to the superclusters found in the SDSS main survey. Results: We find a system that we call the BOSS Great Wall (BGW), which consists of two walls with diameters 186 and 173 h-1 Mpc and two other major superclusters with diameters of 64 and 91 h-1 Mpc. As a whole, this system consists of 830 galaxies with the mean redshift 0.47. We estimate the total mass to be approximately 2 × 1017h-1 M⊙. The morphology of the superclusters in the BGW system is similar to the morphology of the superclusters in the Sloan Great Wall region. Conclusions: The BGW is one of the most extended and massive systems of superclusters found so far in the Universe.