This book constitutes revised selected papers of the 9th International Conference on Mobile Internet Security, MobiSec 2025 held in Sapporo, Japan, December 16–18, 2025.

The 38 full papers presented in this volume were carefully reviewed and selected from 109 submissions. They were grouped into the following topical sections: Advanced Cryptography & Formal Security; Quantum & Post-Quantum Security; AI-driven Security & Machine Learning; Network Security & Threat Detection; 5G/6G & Next-Generation Network Security; IoT, CPS & Industry Security; System & Cloud Security; Blockchain & Distributed Trust.