Background: Pulmonary disorders and poor functional capacity are common complications in patients under hemodialysis. Although breathing exercise is frequently prescribed to improve respiratory function, its efficacy in this patient community is not well established. Purpose: Our study was designed to determine the effectiveness of a breathing exercise on respiratory function and 6-minute walk (6MW) distance in patients under hemodialysis. Methods: A randomized controlled trial approach was used. The sample consisted of 52 patients under hemodialysis from a university teaching hospital in Iran. The experimental group (n = 26) received the breathing exercise program and was encouraged to perform incentive spirometry for 2 months. The control group (n = 26) received only routine hospital care. The respiratory function test and 6MW test were performed at baseline and at 2 months after the intervention (posttest). Results: The two groups were homogeneous in terms of respiratory function parameters, 6MW distance, and demographic characteristics at baseline. Forced expiratory volume in 1 second and forced vital capacity were significantly better in the experimental group compared with the control group at 2 months after intervention. No significant difference was found in 6MW distance between the groups at the 2-month posttest. Conclusions/Implications for Practice: The 2-month breathing exercise effectively improved pulmonary function parameters (forced vital capacity, forced expiratory volume in 1 second) in patients under hemodialysis but did not affect 6MW distance. Hemodialysis nurses should strengthen their clinical health education and apply breathing exercise programs to reduce the pulmonary complications experienced by patients under hemodialysis.