This book constitutes the refereed proceedings of the 23rd International Conference on Group Decision and Negotiation, GDN 2023, which took place in Tokyo, Japan during June 11-15, 2023.The field of Group Decision and Negotiation focuses on decision processes with at least two participants and a common goal but conflicting individual goals. Research areas of Group Decision and Negotiation include electronic negotiations, experiments, the role of emotions in group decision and negotiations, preference elicitation and decision support for group decisions and negotiations, and conflict resolution principles. This year's conference focusses on multimodal interactions. The 11 full papers presented in this volume were carefully reviewed and selected from 102 submissions. They were organized in the following topical sections: Taking a step back: Critically re-examining technology interactions with group decision and negotiation; preference modeling and multi-criteria decision-making; and conflict modeling and distributive mechanisms.