The purpose of this work was to examine biological mechanisms and symptom outcomes of illness uncertainty and psychological stress in Parkinson’s disease (PD). Parkinson’s disease is a chronic, progressive neurodegenerative disorder characterized by complex symptoms that fluctuate in onset, severity, level of disability, and responsiveness to treatment. In addition to characteristic motor symptoms of tremor, rigidity, bradykinesia, and postural instability, a considerable number of individuals with PD also experience debilitating pain, fatigue, and medication-induced motor complications of dyskinesia, dystonia, and on-off phenomena. The unpredictable nature of PD symptoms and motor complications coupled with the inability to halt or slow disease progression may result in uncertainty and psychological stress. Evidence is lacking regarding biological mechanisms and symptom outcomes of uncertainty and psychological stress in PD. As such, 80 men and women diagnosed with PD after the age of 49 were recruited to participate in this study. Data specific to characteristics that may contribute to uncertainty and psychobehavioral measures of uncertainty, appraisal, psychological stress, and symptom outcomes of motor symptoms, pain, and fatigue were collected. Biological measures of neuropeptide Y (NPY) and cytokines were obtained. The results revealed that participants perceived a moderate level of illness uncertainty. Uncertainty correlated significantly with motor symptoms, pain severity, and pain interference and predicted more severe pain severity and pain interference. Psychological stress correlated significantly with motor symptoms, pain severity, pain interference, and fatigue and predicted more severe symptoms across all outcomes. NPY was positively correlated with threat appraisals and psychological stress. Cytokines were below the level of detection in this sample, and not used beyond descriptive analyses. In summary, this study found uncertainty and psychological stress contributed to more severe symptom outcomes in PD. This knowledge may be used to guide future studies aimed at further elucidating biobehavioral symptom and health outcomes of uncertainty and psychological stress in PD. It will also facilitate the development of interventions specifically targeted to uncertainty and psychological stress for the ultimate purpose of improving symptom management, health outcomes, and disease progression in PD.