Recently, a unified stochastic particle (USP) method [14] has been developed for the simulation of multiscale nonequilibrium gas flows. Compared with the conventional stochastic particle methods, such as the direct simulation Monte Carlo (DSMC) method, USP can be applied with much larger time steps and cell sizes as it couples the effects of particle movements and collisions. To extend the applicability of USP to complex nonequilibrium gas flows, we develop a USP solver referred to as SPARTACUS within the framework of SPARTA, which is a widely used software based on the DSMC method for the simulation of rarefied gas flows. Inheriting from SPARTA, SPARTACUS is parallelized with the message passing interface (MPI), and it is open-source under the GNU General Public License (GPL) and released in an online repository with essential documentation, tutorial examples, and benchmark cases. To evaluate the performance of SPARTACUS, the test cases cover from one to three dimensional flows with a wide range of the Knudsen number and Mach number, including Couette flow, lid-driven cavity flow, hypersonic flow past a cylinder, and supersonic flow around a blunt body. The results obtained by SPARTACUS in the rarefied and continuum regimes are in good agreement with DSMC and computational fluid dynamics (CFD) results, respectively. Moreover, SPARTACUS has significant computational efficiency advantage over traditional DSMC solvers, especially for three-dimensional nonequilibrium gas flows. In addition, SPARTACUS shows comparable parallel efficiency with SPARTA, so it is promising to be applied with high performance computing for the simulation of complex multiscale gas flows encountered in engineering problems. Program Title: SPARTACUS CPC Library link to program files: https://doi.org/10.17632/pyb343w7zv.1 Developer's repository link: https://github.com/KKFeng/spartacus Licensing provisions: GNU General Public License version 2 Programming language: C++ External routines/libraries: SPARTA (http://sparta.sandia.gov/) Nature of problem: Multiscale simulation of nonequilibrium gas flows using unified stochastic particle method. Solution method: Unified Stochastic Particle (USP) method. • An open-source unified stochastic particle solver, SPARTACUS, was developed for the simulation of multiscale gas flows. • The results obtained by SPARTACUS in both rarefied and continuum regimes agree well with DSMC or CFD results. • SPARTACUS has significant computational efficiency advantage over traditional DSMC solvers. [ABSTRACT FROM AUTHOR]