This publication includes the Proceedings of the 11th International Symposium on Tunnel Safety and Security (ISTSS) held in Reykjavik, Iceland, April 9-11, 2025. The Proceedings include 43 papers and 17 posters. They were presented in 16 different sessions, i.e., 3 Keynote sessions, Fire Dynamics 1&2, Fixed Fire Fighting Systems, Alternative Fuel Vehicles, Poster session, Evacuation 1&2, Ventilation, Case Studies 1&2, Risk 1&2, and Emergency Management.