This LNCS conference volume constitutes the proceedings of the29th International Conference on Developments in Language Theory, DLT 2025, in Seoul, South Korea, in August 2025. The 18 full papers and 2 invited papers included in this book were carefully reviewed and selected from 38 submissions. They presented current developments in formal languages and automata. Its scope is very general and includes, among others, the following topics and areas: grammars, acceptors and trans-ducers for words, trees and graphs; algebraic theories of automata; algorithmic and many more.