This book constitutes the refereed proceedings of the 6th International Workshop on Lightweight Cryptography for Security and Privacy, LightSec 2025, held in Istanbul, Turkey, during September 1–2, 2025.The 18 full papers presented were carefully reviewed and selected from 31 submissions. The papers were organized into the following topical sections: Post-Quantum Cryptography; Efficient Implementation of Post-Quantum Cryptography; Advanced Cryptographic Algorithms: Fully Homomorphic Encryption, Zero-Knowledge; Hardware and Architecture Security; and Security, Cryptanalysis and Attacks.