This book constitutes the refereed proceedings of the 15th International Conference on Augmented Cognition, AC 2021, held as part of the 23rd International Conference, HCI International 2021, held as a virtual event, in July 2021. The total of 1276 papers and 241 posters included in the 39 HCII 2021 proceedings volumes was carefully reviewed and selected from 5222 submissions. AC 2021 includes a total of 32 papers; they were organized in topical sections named: BCI and brain activity measurement physiological measuring and human performance; modelling human cognition; and augmented cognition in complex environments.