BACKGROUND:A relationship has been observed between physical activity and cognition in older-onset Parkinson's disease, as well as improvements in cognition after a physical activity intervention. To date, this has not been investigated in young-onset Parkinson's disease (YOPD). OBJECTIVES:To examine the baseline relationship between physical activity and cognition in YOPD; and to examine whether a physical activity intervention can improve cognition in YOPD. METHODS:Two interrelated online studies were conducted. In the first study, 132 participants with YOPD completed self-reported measures of physical activity, and objective and subjective measures of cognition. A subset of 38 participants was then randomly allocated to either a six-week physical activity intervention or control condition. Following the intervention, participants repeated the objective and subjective cognitive measures. RESULTS:No relationship was found between self-reported physical activity and objective cognition; however, there was a relationship between physical activity and subjective cognition. Similarly, following the intervention subjective improvements were found for concentration, attention, and processing speed, but not for memory. Furthermore, medium effect sizes were evident for objective measures of processing speed and small-medium effect sizes for planning and cognitive flexibility, although statistical significance was not reached. CONCLUSIONS:In this first study investigating physical activity and cognition in YOPD, the results suggest that increased physical activity relates to improved processing speed and attention. Replication is recommended with a larger sample size. A longer, more intense physical activity manipulation and utilizing the study's strengths of online recruitment and intervention delivery are also recommended.