The objective of this study was to investigate the effect of psychological adjustment to Parkinson's disease (PD) on the health-related quality of life (HRQL) in patients with this condition. One hundred eighty-three patients (77 male, 106 female; mean age, 65.8 years) were evaluated using the Parkinson's Disease Questionnaire (PDQ-39) and the Japanese version of the Nottingham Adjustment Scale (NAS-J). Multiple regression analysis was performed using age and sex as the explanatory variables. Comparisons were made of changes in R2 when severity and psychological adjustment subscales were added. In addition, comparisons were made of HRQL scores between groups with different levels of psychological adjustment. R2 was higher when psychological adjustment was included in comparison with severity in the some subscales (emotional well-being, stigma, social support, cognition, communication, and bodily discomfort). HRQL was significantly lower in the low-adjustment group compared with other groups. Psychological adjustment had a greater effect than severity of disease on several subscales in the HRQL of patients with PD. This finding suggests that, in addition to any suppression of the progression of symptoms, psychological intervention may also be effective in enhancing the HRQL of patients with PD.