We introduce the C++ open-source programming library RINGMesh developed to provide a set of structures and basic tools to manipulate meshes representing geological objects. These objects can be described either by their boundary surfaces or by volumetric meshes. RINGMesh can store several meshes for a given rock unit and can represent faults as continuous or discontinuous surfaces in the volumetric mesh. This makes it possible to experiment with several meshing algorithms, parameters, and physical simulation codes. We provide efficient, documented C++ classes representing meshed geological models, as well as an easy to use application programming interface (API) to import, manipulate, modify, generate, and export in various formats these models. The library is distributed under the modified BSD license and is available at http://ring.gocad.org/index.php/software/ringmesh.