This book constitutes the refereed proceedings of the 13th International Conference on Virtual, Augmented and Mixed Reality, VAMR 2021, held virtually as part of the 23rd HCI International Conference, HCII 2021, 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.The 47 papers included in this volume were organized in topical sections as follows: designing and evaluating VAMR environments; multimodal and natural interaction in VAMR; head-mounted displays and VR glasses; VAMR applications in design, the industry and the military; and VAMR in learning and culture.