This book constitutes revised selected papers from the 21st International Symposium on Trends in Functional Programming, TFP 2020, which was held in Krakow, Poland, during February 13-14, 2020. The 11 full papers presented in this volume were carefully reviewed and selected from 22 submissions. They were organized in topical sections named: domain-specific languages; debugging and testing; reasoning and effects; and parallelism.