This book constitutes the refereed proceedings of the 24th Symposium on Formal Methods, FM 2021, held virtually in November 2021. The 43 full papers presented together with 4 invited presentations were carefully reviewed and selected from 131 submissions. The papers are organized in topical sections named: Invited Presentations. - Interactive Theorem Proving, Neural Networks & Active Learning, Logics & Theory, Program Verification I, Hybrid Systems, Program Verification II, Automata, Analysis of Complex Systems, Probabilities, Industry Track Invited Papers, Industry Track, Divide et Impera: Efficient Synthesis of Cyber-Physical System.