Although aquatic exercise is considered a potentially effective treatment intervention for people with osteoarthritis (OA), previous research has focused primarily on calisthenics in a shallow pool with the inherent limitations on regulating exercise intensity. The purpose of this study was to quantify the efficacy of a 6-week aquatic treadmill exercise program on measures of pain, balance, function, and mobility. Eighteen participants (age = 64.5 ± 10.2 years) with knee OA completed a non-exercise control period followed by a 6-week exercise period. Outcome measures included visual analog scales for pain, posturography for balance, sit-to-stand test for function, and a 10-m walk test for mobility. The exercise protocol included balance training and high-intensity interval training (HIT) in an aquatic treadmill using water jets to destabilize while standing and achieve high ratings of perceived exertion (14-19) while walking. In comparison with pretests, participants displayed reduced joint pain (pre = 50.3 ± 24.8 mm vs. post = 15.8 ± 10.6 mm), improved balance (equilibrium pre = 66.6 ± 11.0 vs. post = 73.5 ± 7.1), function (rising index pre = 0.49 ± 0.19% vs. post = 0.33 ± 0.11%), and mobility (walk pre = 8.6 ± 1.4 s vs. post = 7.8 ± 1.1 s) after participating in the exercise protocol (p = 0.03-0.001). The same benefits were not observed after the non-exercise control period. Adherence to the exercise protocol was exceptional and no participants reported adverse effects, suggesting that aquatic treadmill exercise that incorporates balance and HIT training was well tolerated by patients with OA and may be effective at managing symptoms of OA.