This book constitutes the extended abstracts of the posters presented during the 21st International Conference on Human-Computer Interaction, HCII 2019, which took place in Orlando, Florida, in July 2019. The total of 1274 papers and 209 posters included in the 35 HCII 2019 proceedings volumes was carefully reviewed and selected from 5029 submissions. The 55 papers presented in this volume are organized in topical sections as follows: interaction design; cognitive issues in HCI; accessibility and universal access; learning and games; HCI in health and rehabilitation; HCI in business and society; big data, machine learning and visual analytics; and user studies.