Approximation and online algorithms are fundamental tools to deal with computationally hard problems and problems in which the input is gradually disclosed over time. Both kinds of problems arise from a large number of applications in a variety of fields. The Workshop on Approximation and Online Algorithms (WAOA) focuses on the design and analysis of approximation and online algorithms. It also covers experimental methods used to design and analyze efficient approximation and online algorithms. It is colocated with ALGO 2023, which also hosts ESA, ALGOCLOUD, ALGOSENSORS, ATMOS and IPEC. ALGO 2023 will take place in Amsterdam, the Netherlands.