The Geodynamic World Builder (GWB) is an open source code library intended to set up initial conditions for computational geodynamic models and/or visualize complex 3d teconic setting, in both Cartesian and Spherical geometries. The inputs for the JSON-style parameter file are not mathematical, but rather a structured nested list describing tectonic features, e.g. a continental, an oceanic or a subducting plate. Each of these tectonic features can be assigned a specific temperature profile (e.g. plate model) or composition label (e.g. uniform). For each point in space, the GWB can return the composition and/or temperature. It is written in C++, but can be used in almost any language through its C, Python and Fortran wrappers. Various examples of 2D and 3D subduction settings are presented.