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 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 2021, which also hosts ESA, ALGOCLOUD, ALGOSENSORS and ATMOS. ALGO 2021 will take place 6-10 September 2021 in Lisbon, Portugal.