This book constitutes the proceedings of the 18th International Symposium on NASA Formal Methods, NFM 2026, held in Los Angeles, CA, USA in May 2026.The 23 regular papers and 2 short papers presented in the proceedings were carefully reviewed and selected from 60 submissions. They focus on formalization of natural languagerequirements, contracts, theorem proving and rewriting logic, probabilistic model checking, automated constraint solving, static analysis, runtime verification, testing, automata learning, formal semantics and verification of programming languages, verification of AI systems, AI-assisted formal verification, and large language models (LLMs).