This book constitutes the refereed proceedings of the 27th International Conference on Developments in Language Theory, DLT 2023, held in Umeå, Sweden, during June 12-16, 2023. The 20 full papers included in this book were carefully reviewed and selected from 32 submissions (31 regular ones and one invited). The DLT conference series provides a forum for presenting current developments informal languages and automata. Its scope is very general and includes, among others, the following topics and areas: grammars, acceptors and transducers for words; trees and graphs; relations between formal languages and artificial neural networks; algebraic theories of automata; algorithmic, combinatorial, and algebraic properties of words and languages; variable length codes; symbolic dynamics; cellular automata; groups and semigroups generated by automata; polyominoes and multidimensional patterns; decidability questions; image manipulation and compression; efficient text algorithms; relationships to cryptography, concurrency, complexity theory, and logic; bio-inspired computing; and quantum computing.