This book constitutes the proceedings of the 30th International Conference on Automated Reasoning with Analytic Tableaux and Related Methods, TABLEAUX 2021, held in Birmingham, UK, in September 2021. The 23 full papers and 3 system descriptions included in the volume were carefully reviewed and selected from 46 submissions.They present research on all aspects of the mechanization of tableaux-based reasoning and related methods, including theoretical foundations, implementation techniques, systems development and applications. The papers are organized in the following topical sections: tableau calculi, sequent calculi, theorem proving, formalized proofs, non-wellfounded proofs, automated theorem provers, and intuitionistic modal logics.