UCNS3D is an open-source computational solver for compressible flows on unstructured meshes. State-of-the-art high-order methods and their associated benefits can now be implemented for industrial-scale CFD problems due to the flexibility and highly-automated generation offered by unstructured meshes. We present the governing equations of the physical models employed in UCNS3D, and the numerical framework developed for their solution. The code has been designed so that extended to other systems of equations and numerical models is straightforward. The employed methods are validated towards a series of stringent well-established test problems against experimental or analytical solutions, where the full capabilities of UCNS3D in terms of applications spectrum, robustness, efficiency, and accuracy are demonstrated. Program summary: Program title: UCNS3D (Unstructured Compressible Flow Solver) CPC Library link to program files: https://doi.org/10.17632/222zh873kh.1 Developer's repository link: https://github.com/ucns3d-team/UCNS3D Licensing provisions: GNU General Public License 3 Programming language: Fortran2008 Nature of problem: UCNS3D is intended for the simulation of compressible flows in 2D and 3D unstructured meshes, by employing high-resolution, high-order methods capable of providing physically meaningful results in a computational efficient manner. The solver is designed for a broad range of problems encountered in engineering applications such as transitional, fully turbulent, and multicomponent flows with several fidelity level modelling options available. Solution method: The present software includes multiple physical models, numerical methods, and modelling techniques such as iLES, RANS, DES for unstructured meshes. The software has been developed such that the inclusion of additional physical models and numerical methods can be easily accommodated.