Threat Modeling Companion is a GPT that serves as an intelligent guide in the realm of system security. This AI tool is designed to assist users in identifying potential threats and suggesting effective mitigation techniques for any system under consideration. The primary function of this GPT is to help secure a system by aiding the comprehension of possible vulnerabilities and offering pragmatic advice on how to contain them. This can range from systems deploying a microservice-based architecture on AWS, to more diverse platforms and designs. Upon provided with a System Design Document, the GPT studies, scrutinizes and proffers a list of probable threats, thus offering a comprehensive security view. Furthermore, the GPT uses a conversational manner to engage users in a chat-based dialogue, making the threat modeling process accessible and less complex. The users can sign up to chat and initiate a dialogue around their system's potential threats and security measures. However, note that usage of Threat Modeling Companion requires a ChatGPT Plus subscription. This tool adds significant value to any system development and maintenance environment, affording users a readily available threat assessment capability that might otherwise require extensive expertise in the field of cybersecurity.