This book constitutes the refereed proceedings of the 33rd International Symposium on Logic-Based Program Synthesis and Transformation, LOPSTR 2023, held in Cascais, Portugal, during October 23-24, 2023.  The 8 full papers and 4 short papers included in this book were carefully reviewed and selected from 29 submissions. They were organized in topical sections as follows: Horn Clauses Analysis, Transformation and Synthesis; Static analysis and Type systems; Unification and substitution in (C)LP; and Knowledge representation and AI-based learning.