This book constitutes the refereed proceedings of the 23rd Symposium on Formal Methods, FM 2019, held in Porto, Portugal, in the form of the Third World Congress on Formal Methods, in October 2019. The 44 full papers presented together with 3 invited presentations were carefully reviewed and selected from 129 submissions. The papers are organized in topical sections named: Invited Presentations; Verification; Synthesis Techniques; Concurrency; Model Checking Circus; Model Checking; Analysis Techniques; Specification Languages; Reasoning Techniques; Modelling Languages; Learning-Based Techniques and Applications; Refactoring and Reprogramming; I-Day Presentations.