We tested a self-determination theory (SDT) process model during a 3 week physical activity rehabilitation stay among young adults with a physical disability (N = 44, Mage = 24.7, SD = 5.1). As hypothesized, perceived autonomy support positively predicted needs satisfaction at the end of the stay (r = .38, p < .01). Further, needs satisfaction was positively linked to changes in autonomous motivation for physical activity (r = .47, p < .01). Both changes in autonomous motivation and self-efficacy were associated with physical activity increases over the stay (r = .57, p < .01 and r = .47, p < .01, respectively). Bootstrapping results supported the SDT process model, indicating a support for a development toward more self-determined motivation in rehabilitation.