Electromagnetic radiation is a key component in many astrophysical simulations. Not only does it dictate what we can or cannot observe, it can provide radiation pressure, efficient heating and cooling mechanisms, and opens up a range of new chemical pathways due to photo-reactions. Magritte is a software library that can be used as a general-purpose radiative transfer solver, but was particularly designed for line radiative transfer in complex 3D morphologies, such as, for instance, encountered in the stellar winds around evolved stars. It is mainly written in C++ and can either be used as a Python package or as a C++ library. To compute the radiation field, a deterministic ray-tracer and a formal solver are employed, i.e., rays are traced through the model and the radiative transfer equation is solved along those rays. This is in contrast to most radiative transfer solvers which employ (probabilistic) Monte Carlo techniques. By virtue of minimal assumptions about the underlying geometric structure of a model, Magritte can handle structured and unstructured input meshes, as well as smoothed-particle hydrodynamics (SPH) data. Furthermore, tools are provided to optimise different input meshes for radiative transfer.