Mentally tough behaviors (MTbs) entail verbal or physical acts that allow athletes to engage their capacity to produce consistently high performances under pressure. However, researchers of mental toughness (MT) have typically focused on the characteristics that make an athlete mentally tough, rather than how these characteristics are developed through learning to, and reflecting on, the display of MTbs. Consequently, we explored the athlete MT development process within youth international football. Collaborating with a Union of European Football Associations (UEFA) National Association, age-group international players (n = 6), coaches (n = 6), support staff (n = 7), and parents (n = 6) were interviewed regarding MTbs, the specific contexts (e.g., training) that require MTbs, and how key personnel (e.g., coaches, parents, teammates) might help develop players' MTbs in international youth football. Using thematic analysis, we found MT development to be a relational, multidimensional process, where players transacted with individuals (e.g., coaches) in their environment. These individuals' behaviors (e.g., autonomy-supportive) influenced players' propensity to engage in and reflect on contextually relevant MTbs, leading to MT understanding, development, and maintenance. We suggest organizations develop a common understanding of the MT development process and educate all relevant stakeholders regarding their role in supporting athletes to develop the capacity to perform consistently under pressure.Lay summary: We explored key stakeholders' perceptions of mental toughness (MT) development in international youth football. We found players' MT is developed via key relationships, with players encountering opportunities to display this capacity via mentally tough behaviors (MTbs). Players must reflect upon their display of MTbs to further develop and maintain MT. Practitioners should assist coaches in learning to consistently recognize MTbs in critical moments in training and competition and reinforce the display of these MTbs in players via autonomy-supportive practices.Practitioners should educate athletes regarding the importance of displaying and reflecting on MTbs under pressure to develop their MT capacity.Parents of international players need to be educated on what MT is and the role they play in developing their child's MT at home.
