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, Engineering Case Studies 1&2, Risk 1&2, and Emergency Management.Each day was opened by invited Keynote Speakers (in total six) addressing broad topics of pressing interest. The Keynote Speakers, selected as leaders in their field, consisted of Daniel Joyeux (Efectis Group, France), Haukur Ingason (RISE Research Institutes of Sweden, Sweden), Håkan Frantzich (Lund University, Sweden), Igor Maevski (Jacob, USA), Jie Ji (University of Science and Technology of China, China) and Jennifer Wen (University of Surrey, UK). We are grateful that the keynote speakers were able to share their knowledge and expertise with the participants of the symposium.