The Transfer Matrix Method (TMM) has become a prominent tool for the optical simulation of thin−film solar cells, particularly among researchers specializing in organic semiconductors and perovskite materials. As the commercial viability of these solar cells continues to advance, driven by rapid developments in materials and production processes, the importance of optical simulation has grown significantly. By leveraging optical simulation, researchers can gain profound insights into photovoltaic phenomena, empowering the implementation of device optimization strategies to achieve enhanced performance. However, existing TMM−based packages exhibit limitations, such as requiring programming expertise, licensing fees, or lack of support for bilayer device simulation. In response to these gaps and challenges, we present the TMM Simulator (TMM−Sim), an intuitive and user−friendly tool to calculate essential photovoltaic parameters, including the optical electric field profile, exciton generation profile, fraction of light absorbed per layer, photocurrent, external quantum efficiency, internal quantum efficiency, and parasitic losses. An additional advantage of TMM−Sim lies in its capacity to generate outcomes suitable as input parameters for electro−optical device simulations. In this work, we offer a comprehensive guide, outlining a step−by−step process to use TMM−Sim, and provide a thorough analysis of the results. TMM−Sim is freely available, accessible through our web server (nanocalc.org), or downloadable from the TMM−Sim repository (for Unix, Windows, and macOS) on GitHub. With its user−friendly interface and powerful capabilities, TMM−Sim aims to facilitate and accelerate research in thin−film solar cells, fostering advancements in renewable energy technologies. •Intuitive and user-friendly tool for optical simulation of essential photovoltaic parameters.•Ability to simulate two device architectures: bilayer and bulk heterojunction.•Possibility of calculating the photocurrent as a function of the active layer thickness for device optimization.•The software is freely available for use either through a dedicated web server or by downloading the binary files.