RapidSim is a lightweight application for the fast simulation of phase space decays of beauty and charm quark hadrons, allowing for quick studies of the properties of signal and background decays in particle physics analyses. Based upon the TGenPhaseSpace class from the ROOT application it uses externally provided fixed-order next-to-leading-logarithm calculations to boost the initial beauty and charm hadrons to the appropriate energy for the production environment of interest. User-defined momentum resolution functions can be used to mimic the effect of imperfect track reconstruction. User-defined efficiency shapes can be applied during generation to reproduce the effects of geometric and kinematic requirements on final state particles as well as the dynamics of the decay. The effect of mis-identification of the final state particles is simple to configure via configuration files, while the framework can easily be extended to include additional particle types. This paper describes the RapidSim framework, features and some example use cases. Program Title: RapidSim Program Files doi:http://dx.doi.org/10.17632/62gg6pz6z7.1 Licensing provisions: MIT Programming language: C++ Nature of problem: A common problem in the analysis of particle decays is understanding the kinematic properties of the signal decay of interest and the potential backgrounds that can be introduced via other particle decays that have been imperfectly reconstructed in the detector. In particular, it is often crucial to know the shape of the invariant mass of the final state particles (or sub-sets of them) and the corresponding shape of the background decays. One method to study potential background sources is to generate large samples of the decays and pass them through the full detector simulation and reconstruction software chain. By selecting these events as if they are the signal decay, it is possible to study the shape of the backgrounds in the invariant mass distribution of interest. The disadvantage of this approach is the typically long time required to generate, reconstruct and select these background samples and the mass storage requirements to retain these samples. This long turnaround time can hinder progress of the analysis. Solution method: RapidSim is an application that allows analysts to quickly generate large samples (a few seconds for millions of events) of potential background decays with momentum spectra, invariant mass resolutions and efficiency shapes that are close approximations to what can be obtained from a full detector simulation. The speed of generation allows analysts to quickly perform initial studies that may indicate avenues for further investigation that may need a more detailed simulation.