This study aimed to (a) explore the associations between psychosocial factors and physical activity behavior in people with physical disabilities and/or chronic diseases, both between and within persons over time; and (b) examine whether these associations differ for people initiating and people maintaining physical activity behavior. Research Method/Design: Data of 1,256 adults with physical disabilities and/or chronic diseases enrolled in the prospective cohort study Rehabilitation, Sports, and Active lifestyle (ReSpAct) were analyzed. Self-reported physical activity and four main psychosocial factors (i.e., self-efficacy, attitude, motivation, social support) were measured with questionnaires 3-6 weeks before discharge (T0) and 14 (T1), 33 (T2), and 52 (T3) weeks after discharge from rehabilitation. Hybrid multilevel regression models (corrected for age, sex, education level, diagnosis, counseling support) were used. Results: Multivariable significant Rehabilitation Psychology