This book constitutes the refereed proceedings of the 23rd International Workshop on Approximation and Online Algorithms, WAOA 2025, held in Warsaw, Poland, during September 18–19, 2025.

The 15 full papers included in this book were carefully reviewed and selected from 43 submissions. WAOA 2025 invited submissions on topics related to approximation and online algorithms, including algorithmic game theory, coloring and partitioning,computational economics and mechanism design, experimental methods for approximation and online algorithms, FPT-approximation algorithms, geometric problems, graph algorithms and network design, inapproximability results, packing and covering, matroids and submodular functions, new paradigms in approximation and online optimization, online selection problems, resource augmentation, relaxations and tightness of formulations, robust and stochastic problems,and scheduling problems.