Purpose: In this case report, we examined the effects of an aquatic exercise program on respiratory and knee extensor muscle strength in a 72-year-old female patient with knee osteoarthritis and reduced respiratory muscle strength. Decreased respiratory muscle strength is linked to a diminished coughing capacity, which increases the risk of atelectasis and pneumonia, particularly in older individuals with limited physical activity. This study reports the effects of aquatic exercise on a patient with knee osteoarthritis and decreased respiratory muscle strength.  Methods: The patient participated in an 8-week aquatic exercise regimen, including walking in water at a depth above the fourth intercostal level, to achieve 60% of the predicted maximum heart rate. The pre- and post-intervention measurements included knee extension strength, grip strength, respiratory muscle pressure, vital capacity, and 6-meter walking speed.  Results: Improvements in expiratory muscle strength, bilateral knee extension strength, and walking speed were observed after the intervention.   Conclusions: Buoyancy of the aquatic environment minimizes joint load, whereas water viscosity provides resistance and enhances muscle strength without adverse effects. The increase in expiratory muscle strength suggests a potential training effect on the respiratory muscles owing to hydrostatic pressure resistance. These findings underscore the value of aquatic exercise in patients with musculoskeletal limitations and suggest its utility in maintaining respiratory function in older populations. Future research should explore long-term effects and include broader sample sizes to validate these outcomes.