As one of the Vs defining Big Data, data velocity brings many new challenges to traditional data processing approaches. The adoption of cloud environments in complex event processing (CEP) systems is a recent architectural style that aims to overcome these challenges. Validating cloud-based CEP systems at the required Big Data scale, however, is often a laborious, error-prone, and expensive task. This article presents CEPSim, a new simulator that has been developed to facilitate this validation process. CEPSim extends CloudSim, an existing cloud simulator, with an application model based on directed acyclic graphs that is used to represent continuous CEP queries. Once defined, the queries can be simulated in different cloud environments under diverse load conditions. Moreover, CEPSim is also customizable with different operator placement and scheduling strategies. These features enable researchers and system architects to experiment with different configurations and strategies and to promote research in this field. Experimental results show that CEPSim can successfully simulate existing cloud-based CEP systems.