Post-K is the next flagship supercomputer in Japan, replacement of the K supercomputer. Its node architecture and interconnect are based on Armv8 SVE and a 6-D mesh/torus network, respectively. A three level hierarchical storage system will be installed with compute nodes. The system software developed in the post K supercomputer includes a novel operating system for general-purpose manycore architectures, low-level communication and MPI libraries, and file I/O middleware.