Young adults with autism spectrum disorder are an underserved population. Evidence-based guidelines for physical activity programs to address the lifelong needs of this population are presently unavailable. Our systematic review critically appraises the research to date on the effects of physical activity/exercise-based interventions on physical fitness, motor skills, core autism symptoms, and functional participation in young adults with autism spectrum disorder between 19 and 30 years. We reviewed 22 experimental and quasi-experimental studies that assessed the effects of physical activity/exercise-based interventions in 763 young adults with autism spectrum disorder. Evidence is strongest for improvements of medium-to-large size in physical fitness followed by medium-to-large improvements in motor outcomes, medium-to-large improvements in psychological function, and small-to-large improvements in quality of life. Aerobic, resistance, and movement skill/sport-specific training can improve cardiovascular function and muscular strength/endurance in young adults. Movement/sport training can help improve fundamental motor skills. Finally, holistic interventions combining elements of physical activity, diet, and lifestyle modifications are effective in improving outcomes related to body composition and quality of life in young adults. There is presently insufficient evidence for improvements in physical activity engagement or core autism symptoms following exercise interventions. We provide recommendations for assessment and intervention for clinicians working with young adults with autism spectrum disorder. Lay abstract Young adults with autism spectrum disorder over 18 years of age are an underserved population, and there is presently limited evidence examining the effects of physical activity programs in this population. Our review synthesizes the evidence to date from studies that have assessed the effects of physical activity/exercise programs in young adults with autism spectrum disorder between 19 and 30 years. We reviewed 22 studies that included a total of 763 young adults with autism spectrum disorder. There is the strongest evidence for improvements in physical fitness, followed by motor skills, psychological function, and quality of life following physical activity interventions in young adults with autism spectrum disorder. Specifically, aerobic and resistance training as well as programs focused on movement skill and sport-specific training lead to improved physical fitness and movement performance. Holistic interventions focusing on physical activity, dietary changes, and lifestyle modifications lead to improvements in body composition and quality of life of young adults with autism spectrum disorder. There is presently limited evidence to support the use of exercise/activity programs to improve physical activity levels and core autism symptoms in young adults with autism spectrum disorder. Based on our review results, we also provide practical recommendations for clinicians working with young adults with autism spectrum disorder.