This book constitutes the refereed post-conference proceedings of the 28th International Workshop on Functional and Constraint Logic Programming, WFLP 2020, held in Bologna, Italy, in September 2020.Due to the COVID-19, the workshop was held online. From the 19 full papers submitted, 8 were accepted for presentation at the workshop. The accepted papers cover different programming areas of functional and logic programming, including code generation, verification, and debugging.