We introduce Sb2Si2Te6 as a high-performance thermoelectric material. Single-crystal X-ray diffraction analysis indicates that Sb2Si2Te6 has a layered two-dimensional structure with Sb3+ cations and [Si2Te6](6) units as building blocks adopting the Fe2P2Se6 structure type. Sb2Si2Te6 is a direct-band-gap semiconductor with valence-band maximum and conduction-band minimum at the Z point in the Brillouin zone, where the band is doubly degenerate. Polycrystalline bulk pellets of Sb2Si2Te6 with randomly packed grains exhibit an intrinsically high thermoelectric figure of merit ZT of similar to 1.08 at 823 K. We then create a cellular nanostructure with ultrathin Si2Te3 nanosheets covering the Sb2Si2Te6 grains, which act as a hole-transmitting electron-blocking filter and at the same time cause extra phonon scattering. This dual function of the cellular nanostructure achieves an ultralow thermal conductivity value of similar to 0.29 Wm(-1) K-1 and a high ZT value of similar to 1.65 at 823 K for Sb2Si2Te6, along with a high average ZT value of similar to 0.98.