This study investigates the impact of group cognitive behavioral therapy (CBT) on depression reduction and quality of life improvement among individuals with Parkinson disease. A randomized clinical trial with pretest and posttest measurements involved 90 participants referred to Roozbeh Hospital in Tehran in 2023, who were randomly assigned to either an experimental group (n = 45) or a control group (n = 45). The experimental group underwent a 3-month CBT intervention comprising 12 sessions of 90 minutes each. Both groups completed the Beck Depression Inventory and the World Health Organization Quality of Life Questionnaire preintervention and postintervention. Data analysis via multivariate analysis of covariance using SPSS-25 revealed significant improvements in reducing depression and enhancing quality of life in the experimental group compared with the control group (p ≤ 0.01). These findings indicate that CBT is an effective treatment for reducing depression and improving the quality of life of individuals with Parkinson disease, demonstrating its clinical applicability in therapeutic settings.