This book constitutes the refereed proceedings of the 21st Asian Symposium on Programming Languages and Systems, APLAS 2023, held in Taipei, Taiwan, during November 26–29, 2023. The 15 full papers included in this book are carefully reviewed and selected from 32 submissions. They were organized in topical sections as follows: semantics, logics, and foundational theory; design of languages, type systems, and foundational calculi; domain-specific languages; compilers, interpreters, and abstract machines; program derivation, synthesis, and transformation; program analysis, verification, and model-checking; logic, constraint, probabilistic, and quantum programming; software security; concurrency and parallelism; tools and environments for programming and implementation; and applications of SAT/SMT to programming and implementation.