This book constitutes the refereed post-conference proceedings of the 26th International Workshop on Functional and Constraint Logic Programming, WFLP 2018, held in Frankfurt/Main, Germany, in September 2018.  From the 19 full papers submitted,12 were accepted for presentation at the workshop. During a second review after the workshop, 10 papers were accepted for inclusion in this volume. The papers cover different programming areas of functional and logic programming, including code generation, verification, and debugging. They are organized in the following topical sections: compilers and code generation; debugging and testing; and foundations of functional logic programming.