This book constitutes the refereed proceedings of the 31st International Conference on Logic-Based Program Synthesis and Transformation, LOPSTR 2021, which was held during September 7-8, 2021.*The 8 papers presented in this volume were carefully reviewed and selected from a total of 15 reviewed submissions. Additionally, the book also contains 1 full-length invited talk.*Conference was held as a hybrid event due to the COVID-19 pandemic.