Background: The social identity approach proposes that the more older adults identify with the social group of “older adults,” the more they will conform to what they perceive as being normative exercising for their group. However, so far, it remains unclear why older adults adhere to these norms. Objective: This study evaluated whether perceived exercise norms are associated with higher levels of autonomous motivation according to the self-determination theory and actual exercise participation. Methods: A cross-sectional survey, either by regular mail or online, was conducted among 409 older adults in Flanders (Belgium). Results: Our analyses revealed that older adults who perceived more positive older adult norms for exercising were more autonomously motivated to exercise. In explaining 24% of their exercise motivation, older adults’ perceptions of the exercise norms for older adults predicted 6% of their exercise participation. Conclusion: Our findings suggest that social identity approach and self-determination theory can be meaningfully integrated.