Objective To examine the motivational processes underpinning Australian older adults’ physical activity. Method Adults aged 65 years and over (N = 213) completed a questionnaire assessing autonomous motivation and theory of planned behaviour variables (attitudes, subjective norm, and perceived behavioural control), and a 2‐week follow‐up of their self‐reported physical activity (n = 165). Results In a structural equation model, effects of autonomous motivation on intention were mediated by attitudes and perceived behavioural control, but not subjective norm. The model explained 60% and 46% of the variance in intention and physical activity, respectively. Conclusion The findings add to current knowledge of motivational processes underpinning physical activity in older adults. Current results indicate that interventions where the value of regular physical activity can be internalised by offering activities that foster feelings of competence, autonomy, and relatedness may be important to consider in increasing physical activity levels of older adults, a group at risk of sedentary behaviours.