Athletes experience a number of within-career transitions that expose them to a multitude of demands. The club-to-international transition (CIT) is one transition that has received minimal attention. Through cognitive-motivational-relational-theory (CMRT; Lazarus, 1999), we sought to address this gap by exploring the psychosocial demands, and their situational properties, football (soccer) players experience during the CIT. Fourteen age-group international players, and 10 coaches (four club; six international) were interviewed. Using thematic analysis, a range of performance (e.g., competition intensity), organisational (e.g., new organisational culture), and personal demands (e.g., evolving identity), and situational properties (e.g., novelty, ambiguity) were identified. Further, the CIT was perceived as a unique adversity, due to its fluctuating and ambiguous nature. For example, international selection is never guaranteed and is predicated on current performance at club and international level. To positively negotiate this transition, we suggest players need to develop key psychological resources (e.g., mental toughness, resilience) and rely on organisational relationships (e.g., clear feedback processes), which assist them in taking ownership over their development. Our research has worldwide reach through offering international level organisations novel insights to help support players making the CIT and facilitate bespoke interventions that will positively impact both individual player development and long-term performance success.