This book constitutes the refereed proceedings of the 25th International Conference on Group Decision and Negotiation, GDN 2025, which took place in Zaragoza, Spain, during June 2025. 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.  The 12 full papers included in these proceedings were carefully reviewed and selected from 99 submissions. They were organized in topical sections as follows: AI, Ethics, and Societal Impact in Group Decision and Negotiation; Preference Modeling, Evaluation, and Decision Support in Group Contexts; and Conflict Modeling in Complex Decision Environments.