We present the {\tt C++} program \texttt{RelExt} for Standard Model (SM) extensions that feature a Dark Matter (DM) candidate. The tool allows to efficiently scan the parameter spaces of these models to find parameter combinations that lead to relic density values which are compatible with the measured value within the uncertainty specified by the user. The code computes the relic density for freeze-out (co-)annihilation processes. The user can choose between several pre-installed models or any arbitrary other model featuring a discrete $\mathbb{Z}_2$ symmetry, by solely providing the corresponding \texttt{FeynRules} model files. The code automatically generates the required (co-)annihilation amplitudes and thermally averaged cross sections, including the total widths in the $s$-channel mediators, and solves the Boltzmann equation to determine the relic density. It can easily be linked to other tools like e.g.~\texttt{ScannerS} to check for the relevant theoretical and experimental constraints, or to \texttt{BSMPT} to investigate the phase history of the model and possibly related gravitational waves signals.