This book constitutes the thoroughly refereed workshop proceedings of the 11th International Workshop on Approximation and Online Algorithms, WAOA 2013, held in Sophia Antipolis, France, in September 2013 as part of the ALGO 2013 conference event. The 14 revised full papers presented were carefully reviewed and selected from 33 submissions. They focus on the design and analysis of algorithms for online and computationally hard problems, for example in algorithmic game theory, algorithmic trading, coloring and partitioning, competitive analysis, computational advertising, computational finance, cuts and connectivity, geometric problems, graph algorithms, inapproximability results, mechanism design, natural algorithms, network design, packing and covering, paradigms for the design and analysis of approximation and online algorithms, parameterized complexity, real-world applications, scheduling problems.