With the advent of the smart industry, Industrial Control Systems (ICS) moved from isolated environments to connected platforms to meet Industry 4.0 targets. The inherent connectivity in these services exposes such systems to increased cybersecurity risks. To protect ICSs against cyberattacks, intrusion detection systems (IDS) empowered by machine learning are used to detect abnormal behavior of the systems. Operational ICSs are not safe environments to research IDSs due to the possibility of catastrophic risks. Therefore, realistic ICS testbeds enable researchers to analyze and validate their IDSs in a controlled environment. Although various ICS testbeds have been developed, researchers’ access to a low-cost, extendable, and customizable testbed that can accurately simulate ICSs and suits security research is still an important issue. In this paper, we present ICSSIM, a framework for building customized virtual ICS security testbeds in which various cyber threats and network attacks can be effectively and efficiently investigated. This framework contains base classes to simulate control system components and communications. Simulated components are deployable on actual hardware such as Raspberry Pis, containerized environments like Docker, and simulation environments such as GNS-3. ICSSIM also offers physical process modeling using software and hardware in the loop simulation. This framework reduces the time for developing ICS components and aims to produce extendable, versatile, reproducible, low-cost, and comprehensive ICS testbeds with realistic details and high fidelity. We demonstrate ICSSIM by creating a testbed and validating its functionality by showing how different cyberattacks can be applied.