Introduction: Parkinson’s disease (PD) is a chronic neurodegenerative disease of the brain, characterized by motor symptoms–tremor, rigidity, bradykinesia, slowness/smallness, and postural instability– as well as non-motor symptoms including anxiety, depression, sleep disorders, and cognitive deficits. The average age of onset for PD is 60, with earliest patients diagnosed at age 18. One out of 100 people over age 60 have PD. PD patients’ symptoms increase over time and medication does not slow down the progression of PD. Physical activity (PA) is one lifestyle behavior that may slow the progression of the disease and improve the quality of life of PD patients by maintaining their ability to accomplish functional activities of daily living and preserve their independence. However, knowledge of the motivational factors associated with PA in PD patients remains limited. Methods: The current study aimed to i) explore the relationship of select Social Cognitive Theory (SCT) constructs: self-efficacy (SE), outcome expectations (OE), and self-regulation (SR) with PA and health-related quality of life (HRQoL); ii) explore the relationship between PA and HRQoL; and iii) determine if SCT constructs mediate the relationship between PA and HRQoL in PD patients. Results: In this online cross sectional survey of 500 idiopathic PD patients, participants self-reported an average of just over 200 minutes of moderate to vigorous physical activity per week. SE and SR were the most significant predictors of PA. SE and OE were predictive of physical HRQoL, and the addition of BMI, age, Hoehn and Yahr Score, and total number of comorbidities more than doubled the amount of variance explained. To a smaller extent, SE, OE, and SR were predictive of mental HRQoL. SCT correlates mediated the relationship of PA to HRQoL. Discussion: The study population represented a population of PD patients with a high interest in physical activity. Self-reported average weekly moderate-to-vigorous physical activity (MVPA) was much higher than expected. Future studies should attempt to validate MVPA with some type of exercise monitor that would not be sensitive to tremor or other PD specific considerations. Analysis of self-regulation subscales may provide insight into why SR was predictive of physical HRQoL when modeled alone, but not with the other SCT correlates. A deeper evaluation of outcome expectation subscales might also provide a further explanation of why OE was predictive of MVPA when modeled alone, but not with SE and SR. Physical activity was a significant predictor of both mental and physical HRQoL. The covariates BMI, age, Hoehn and Yahr Score, GDS depression score, and total number of comorbidities significantly added to the explanatory power of the relationship between PA and physical HRQoL. These factors should be considered both potential mediators and moderators in future studies in the PD population.