An open-source nonequilibrium emission and absorption infrared line-by-line code is presented. RADIS is built around the HITRAN, HITEMP and CDSD databases for molecules in their electronic ground state. Energy levels are read from tabulated databases or calculated from Dunham expansions. Boltzmann, Treanor, and arbitrary vibrational distributions can be generated. In its present version, nonequilibrium spectra can be computed for CO2 and CO, and equilibrium spectra can be computed for all species of the HITRAN database. New species can be added without modification to the core code. Several optimization strategies are described that reduce computation times of high-temperature, high-resolution spectra with millions of lines down to less than a minute. Additionally, RADIS includes a line survey tool, a built-in look-up database to reduce convergence times when fitting experimental spectra, and a multi-slab module to simulate line-of-sight experiments. Validation cases against existing spectral codes and experimental results from various plasma sources are presented. Finally, RADIS is applied to improve the modeling of the radiation from a plasma torch and a nonequilibrium expansion-tube experiment. RADIS is distributed under LGPLv3 license .