This book constitutes the thoroughly refereed revised selected papers of the 20th International Symposium on Trends in Functional Programming, TFP 2019, held in Vancouver, Canada, in June 2019. The 6 revised full papers were selected from 11 submissions and present papers in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions, described in draft papers submitted prior to the symposium.