A self-determination theory process model of change in well-being was tested among older adults in a 16-week randomized exercise-trial (n = 118, M = 74.3 years, SD = 4.6, 68% females). The exercise intervention increased participants' autonomous motivation and perceived competence over the first 7 weeks. Moreover, autonomous orientation gave rise to perceived competence, while impersonal orientation was related to increased controlled motivation. Changes in motivational variables were positively related to changes in vitality over the trial, while change in controlled motivation was negatively related to changes in vitality and subjective well-being. Bootstrapping analyses supported an indirect exercise–vitality path through autonomous motivation and perceived competence, and an indirect autonomous orientation–vitality path through perceived competence.