As the Internet of Things (IoT) ecosystem expands, the need for secure and private communication over public networks becomes increasingly crucial. A fundamental component of this security paradigm is the establishment of a secret session key among IoT devices, enabling the secure transmission of confidential data over the Internet. In this paper, we introduce a Post-Quantum Secure Lightweight Authenticated Lattice-based Key Agreement (PQS-AKA) protocol, specifically developed for the rapidly evolving IoT ecosystem. Traditional Authenticated Key Agreement (AKA) protocols, predominantly based on concepts like Diffie–Hellman or prime factorization, are becoming obsolete due to their vulnerability to quantum computing advancements and their inefficiency in the resource-constrained IoT environment. Current quantum-resistant methods, although leveraging lattice cryptography for security, are impeded by issues such as high certificate management overheads and substantial communication and computational costs. PQS-AKA not only offers a defense against the threats posed by quantum computing through the inherent hardness of lattice problems but also simplifies the process by removing the need for extensive certificate management. The core security strength of our protocol is thoroughly evaluated under the random-natured oracle model, demonstrating its robustness against both present and future quantum computing attacks. Performance evaluations reveal that the proposed protocol offers a significant improvement over existing solutions in terms of efficiency and practicality, making it a viable and secure choice for industrial IoT applications.