Background <p>The aquatic environment has characteristics that favor adherence to exercise programs among older adults, and studies have shown that programs in this setting are effective in enhancing various aspects of physical fitness, as well as cognitive function, in this population. Research has explored whether incorporating cognitive tasks into an aquatic exercise program could offer additional benefits. Still, it remains uncertain whether this approach leads to greater improvements in cognitive function compared to aquatic exercise alone. We herein report the protocol of the WaterCog Study, which aims to evaluate the effects of an aquatic aerobic exercise program combined with cognitive tasks, compared to a conventional aquatic aerobic exercise program and a control group, on cognitive function and other health-related outcomes in older women.</p> Methods <p>This trial is a randomized, single-blinded, three-arm, parallel, superiority trial. A total of 102 older women will be randomized into one of three groups: 1) an aquatic aerobic exercise program combined with cognitive tasks, 2) a conventional aquatic aerobic exercise program, and 3) a control group. Participants in both exercise groups will complete a 12-week exercise program consisting of two non-consecutive sessions per week. The primary outcome is cognitive function, while secondary outcomes include physical function, cardiovascular, and psychosocial parameters. Outcomes will be measured at baseline, post-intervention, and the 12-week follow-up after the end of the intervention period. The analysis plan will employ an intention-to-treat approach and per-protocol criteria.</p> Discussion <p>Our conceptual hypothesis is that both aquatic exercise interventions will significantly improve the investigated outcomes compared to the control group. Additionally, we expect that the integration of cognitive tasks will result in additional benefits in cognitive function, cardiovascular outcomes, and psychosocial parameters, with similar gains in physical function compared to conventional aquatic aerobic exercises in post-intervention and follow-up measures. The findings will contribute to the evidence base for interventions targeting cognitive aging and the development of more effective and engaging exercise programs for older adults.</p> Trial registration <p>The trial “Aquatic Exercises Combined with Cognitive Tasks for Older Adults (WaterCog)” was prospectively registered at ClinicalTrials.gov (NCT07156708; <a href="https://clinicaltrials.gov/study/NCT07156708">https://clinicaltrials.gov/study/NCT07156708</a>) in August 2025.</p>