Non-Terrestrial Networks (NTNs) are critical enablers of ubiquitous connectivity in future 6G systems, but they also face challenges such as long propagation delays, significant Doppler effects, and diverse channel conditions. Developing and testing communication technologies for NTNs requires realistic and flexible simulation tools. In this work, we present OpenNTN, an open-source software framework for simulating NTN channel models based on the 3GPP TR38.811 standard. OpenNTN is de- signed as an extension to the Python-based Sionna™ framework, providing channel models compatible with existing interfaces, enabling the use of the powerful tools found in Sionna™ . The framework offers a flexible user interface, enabling researchers to investigate diverse scenarios. As an open-source implementation, OpenNTN empowers the research community to fully use and adapt the framework, offering a solution for both fast and easy NTN research using realistic channel models, while also providing the opportunity to extend the model for custom research interests beyond the current state of the standards.