In humanitarian settings, mental health and psychosocial support services (MHPSS) are often delivered in group-based formats. Group interventions enable providers to reach more individuals when resources and technical expertise are limited. Group-based programs also foster social support, empathy, and collective problem-solving among the participants. To remedy the current lack of tools available to assess the group facilitation competencies of individuals delivering group-based MHPSS, we made it our objective to develop such a tool. Our approach, which focused on adults, complimented a similar initiative underway for children and adolescents. We reviewed MHPSS manuals to identify key group facilitation competencies, which include developing and reviewing group ground rules, facilitating participation among all group members, fostering empathy between members, encouraging collaborative problem-solving, addressing barriers to attendance, time management, and ensuring group confidentiality. We then developed the Group Facilitation Assessment of Competencies (GroupACT) Tool. The GroupACT is a structured observational tool for assessing these competencies during standardized role-plays with actor clients, or in vivo during the delivery of group sessions with actual clients. We conclude this article with guidance for using the GroupACT to assess facilitators' competencies in providing group-based MHPSS in the health, education, protection, and other sectors in humanitarian settings.