Modular steel buildings (MSBs), which are constructed using prefabricated volumetric modules, are increasingly acknowledged as an alternative solution to meet immediate housing needs. These modules are manufactured offsite and then assembled onsite to create multi-story structures. Modular structures inherently possess complex characteristics in their 3D numerical models such as separate diaphragms for each unit and vertical and horizontal connections between the units. Therefore, the development of a Python-based tool, ModularBuildingPy, is an important step in managing these inherent model complexities. It not only minimizes potential modeling errors through automation of the connection details and consistent application of modeling rules, but also accelerates the creation of numerical models, generating them quickly. Moreover, through integration with OpenSeesPy, the tool can effectively perform structural analysis and obtain the structural response (e.g., base shear and inter-story drift values). This provides valuable insights into the structural behavior of the MSBs under various loading conditions such as gravity and seismic. The simulation results can guide design decisions, contributing to the overall safety and serviceability of these structures. To demonstrate the effectiveness and versatility of the Python-based tool, two case studies were conducted focusing on both low- and high-rise MSBs. These case studies provide practical examples of how the tool can be used to generate numerical models quickly and accurately, and to extract results from the analyses. The results from these case studies emphasize the tool’s potential in facilitating the modeling and analysis of MSBs of varying dimensions and complexities.