This volume constitutes the revised selected papers of the 12th International Conference on Modelling and Simulation for Autonomous Systems, MESAS 2025, held on October 21–23, 2025, in Istanbul, Turkey.The 21 full papers included in these proceedings were carefully reviewed and selected from 32 submissions. They were organized in the following topical sections:AxS/AI in Context of Future Warfare and Security Environment (Concepts, Applications, Standards, etc.); M&S of Intelligent Systems – R&D and Application; Future Challenges of Advanced M&S Technology.