This book constitutes the refereed proceedings of the 22nd International Workshop on Approximation and Online Algorithms, WAOA 2024, held in Egham, UK, during September 5-6, 2024. The 15 full papers included in this book were carefully reviewed and selected from 47 submissions. They were organized in topical sections as follows: algorithmic game theory, algorithmic trading, coloring and partitioning, competitive analysis, computational advertising, computational finance, cuts and connectivity, FPT approximation algorithms, geometric problems, graph algorithms, inapproximability results, mechanism design, network design, packing and covering, paradigms for designing and analyzing approximation and online algorithms, resource augmentation, and scheduling problems.