This book constitutes the thoroughly refereed post-conference proceedings of the 27th International Symposium on Logic-Based Program Synthesis and Transformation, LOPSTR 2017, held in Namur, Belgium, in October 2017. The 19 revised full papers were carefully reviewed and selected from 29 submissions. In addition to the 19 revised papers, this volume includes the abstracts of the invited talks by three outstanding speakers: Sumit Gulwani, Marieke Huisman, and Grigore Roşu. The aim of the LOPSTR series is to stimulate and promote international research and collaboration on logic-based program development. LOPSTR is open to contributions in all aspects of logic-based program development, all stages of the software life cycle, and issues of both programming-in-the-small and programming-in-the-large. LOPSTR traditionally solicits contributions, in any language paradigm, in the areas of synthesis, specification, transformation, analysis and verification, specialization, testing and certification, composition, program/model manipulation, optimization, transformational techniques in SE, inversion, applications, and tools.