Abstarct: Introduction Parkinson's disease is caused by deterioration of dopamine producing cells at the brain's basal ganglia, which can adversely affect patients' motor and functional performance especially in elderly people. The purpose of this study is to examine the effect of eight weeks of HIT exercises in water on functional fitness in people with Parkinson's disease. Material and Methods: This study is quasi-experimental with pre-test, post-test, and a control group. The statistical population of the study is made up of all patients with Parkinson's disease visiting neurologists in Isfahan. Forty-eight patients were purposefully and voluntarily selected and then randomly assigned to either the experimental or the control group. The subjects of HIT exercises for eight weeks practiced the exercises three times a week; however, the control group were only under observation during this time and received only the medical treatment without any extra effective physical activities. At the beginning and end of the experiment, balance, upper and lower body strength, upper body and lower body flexibility and the walking speed were assessed respectively through Berg balance test, arm curl test, 30s chair stand test, chair sit and reach test, back scratch test, and Gait speed test. The resulted data were analysed through paired t-test and analysis of covariance (ANCOVA) at p<0.05. Result All factors in the experimental group were improved after complementing the exercise protocol (p<0.05), whereas the control group showed no significant change.