Using a qualitative design, we explored how life transitions influence Danish adults' physical activity beliefs and behaviour adopting the Theory of Planned Behaviour as a guiding framework. Life story interviews with 42 participants revealed 6 salient life transition periods which affected physical activity behaviour, either in a disruptive way resulting in decreased activity levels or in a facilitative manner, providing opportunities for increased activity engagement. The identified life transition periods uniquely influenced individuals' attitudes, subjective norms and perceived behavioural control (PBC) beliefs, thus affecting motivations toward physical activity. Specifically, starting primary school or a career, forming a romantic relationship, and having a child primarily shaped subjective norm and PBC, while leaving home was especially influential on PBC and retirement on attitudes and PBC. Consistent themes across the transition periods included time constraints and opportunities to be active, supportive social contexts, access to facilities and having a need to be active.