This article introduces PyRSB, a Python interface to the LIBRSB library. LIBRSB is a portable performance library offering so called Sparse BLAS (Sparse Basic Linear Algebra Subprograms) operations for modern multicore CPUs. It is based on the Recursive Sparse Blocks (RSB) format, which is particularly well suited for matrices of large dimensions. PyRSB allows LIBRSB usage with an interface styled after that of SciPy’s sparse matrix classes, and offers the extra benefit of exploiting multicore parallelism. This article introduces the concepts behind the RSB format and LIBRSB, and illustrates usage of PyRSB. It concludes with a user-oriented overview of speedup advantage of rsb_matrix over scipy.sparse.csr_matrix running general sparse matrix-matrix multiplication on a modern shared-memory computer.