This book constitutes revised selected papers from the 6th International Workshop on Active Inference, IWAI 2025, held in Montreal, Canada, during October 15-17, 2025.  The 24 full papers included in this book were carefully reviewed and selected from 76 submissions. They were organized in topical sections as follows: Models of Decision Making; Forney Style Factor Graphs; Fundamental Active Inference Extensions; Simulations of Mental Processes; Human Experiments; Scaling and Efficiency; Robotics; Multi-agent and Theory of Mind.