This book constitutes the proceedings of the 19th Asian Symposium on Programming Languages and Systems, APLAS 2021, held in Chicago, USA, in October 2021.* The 17 papers presented in this volume were carefully reviewed and selected from 43 submissions. They were organized in topical sections named: analysis and synthesis, compilation and transformation, language, and verification. * The conference was held in a hybrid format due to the COVID-19 pandemic.