Purpose This research was conducted to determine the effects of an aquatic exercise program on pain, stiffness, physical function, and self-efficacy in individuals with osteoarthritis. Design A randomized controlled trial. Methods Participants in the experimental group participated in the aquatic exercise program three times a week for 8 weeks; participants in the control group did not. Findings The mean scores of the experimental group on the pain, stiffness, and difficulty in carrying out physical functions subscales of the Western Ontario and McMaster Universities Osteoarthritis Index decreased significantly, whereas those of the control group decreased very little. The mean scores of the experimental group on the Arthritis Self-Efficacy Scale and the isokinetic muscle strength measurements increased, but those of the control group did not change in the final measurements. The difference between the groups was statistically significant. Conclusions Through the study, it was determined that the aquatic exercise program decreased pain, stiffness, and difficulty in carrying out physical functions and increased self-efficacy and muscle strength of individuals with osteoarthritis.