  Resonant lines are powerful probes of the interstellar and circumgalactic
medium of galaxies. Their transfer in gas being a complex process, the
interpretation of their observational signatures, either in absorption or in
emission, is often not straightforward. Numerical radiative transfer
simulations are needed to accurately describe the travel of resonant line
photons in real and in frequency space, and to produce realistic mock
observations. This paper introduces RASCAS, a new public 3D radiative transfer
code developed to perform the propagation of any resonant line in numerical
simulations of astrophysical objects. RASCAS was designed to be easily
customisable and to process simulations of arbitrarily large sizes on large
supercomputers. RASCAS performs radiative transfer on an adaptive mesh with an
octree structure using the Monte Carlo technique. RASCAS features full MPI
parallelisation, domain decomposition, adaptive load-balancing, and a standard
peeling algorithm to construct mock observations. The radiative transport of
resonant line photons through different mixes of species (e.g. \ion{H}{i},
\ion{Si}{ii}, \ion{Mg}{ii}, \ion{Fe}{ii}), including their interaction with
dust, is implemented in a modular fashion to allow new transitions to be easily
added to the code. RASCAS is very accurate and efficient. It shows perfect
scaling up to a minimum of a thousand cores. It has been fully tested against
radiative transfer problems with analytic solutions and against various test
cases proposed in the literature. Although it was designed to describe
accurately the many scatterings of line photons, RASCAS may also be used to
propagate photons at any wavelength (e.g. stellar continuum or fluorescent
lines), or to cast millions of rays to integrate the optical depths of ionising
photons, making it highly versatile.
