The three-volume set CCIS 2319-2321 constitutes the proceedings of the 26th International Conference on Human-Computer Interaction, HCII 2024, held in Washington, DC, USA, during June 29-July 4, 2024. For the HCII 2024 proceedings, a total of 1271 papers and 309 posters was carefully reviewed and selected from 5108 submissions. Additionally, 222 papers and 104 posters are included in the volumes of the proceedings published after the conference, as "Late Breaking Work". The posters presented in these three volumes are organized in the following topical sections: Part I: User Interface and Interaction Design; Usability and User Experience Evaluation; Innovative Technologies and Human-Centered Solutions. Part II: Innovations in Extended Reality; Smart Systems and Intelligent Design; AI and Design for Human-Centric Applications. Part III: Design for Health and Well-being; Advanced Interactive Technologies for Learning; Gaming, Gamification, and Immersive Design; Technology-Enhanced Experiences in Cultural Heritage.