In this paper, we propose Adaptive Trust-Based Continuous Authentication Framework (ATCAF), a lightweight architecture to extend Zero Trust principles with continuous risk aware authentication. The proposed framework does not make a single authentication decision at login but continuously calculates a dynamic trust score by combining multiple contextual security indicators such as device trust, behavioral consistency, location confidence, network reputation and historical user reputation. Access decisions are made in real-time based on the continuously evolving trust score, enabling adaptive responses such as seamless access, step-up multi-factor authentication, privilege reduction or session termination. The paper describes the system architecture, mathematical trust model, continuous evaluation workflow, implementation methodology, public evaluation datasets, security analysis and experimental design to evaluate authentication accuracy, False Acceptance Rate (FAR), False Rejection Rate (FRR), Equal Error Rate (EER), trust score stability and response latency . The framework we propose aims at improving identity security, while maintaining usability and reducing unnecessary authentication interruptions.