Objective <p>Non-invasive brain stimulation has been shown to benefit cognitive function in older adults. However, the effects of transcranial alternating current stimulation (tACS) on cognitive function in older adults have not been consistent across studies and there are limitations in the evidence for tACS. This study aims to investigate whether tACS improves cognitive function and different cognitive domains (e.g. learning memory and executive function) in older adults and older adults with Alzheimer’s disease (AD).</p> Methods <p>A systematic search was conducted in seven databases (PubMed, Web of Science, Embase, The Cochrane Library, Scopus, PsycINFO, and CINAHL) up to 25 December 2024. Effect sizes were expressed as standardized mean differences (SMD) with 95% confidence intervals, and a random-effects model was applied.</p> Results <p>Fourteen randomized controlled trials (RCTs) were included, with a relatively low risk of bias. tACS had a significant effect on immediate memory (SMD = 0.66, p&lt;0.01) and delayed memory (SMD = 0.68, p&lt;0.01), but no significant effect on associative memory (SMD =−0.25, p = 0.61). Subgroup analyses revealed that tACS intervention significantly improved immediate and delayed memory in older adults with AD, but had no significant effect on healthy older adults.</p> Conclusion <p>tACS intervention significantly improves immediate memory and delayed memory in older adults with AD. Further large-scale RCTs are needed to clarify the specific effects of tACS on various cognitive domains, and optimal stimulation parameters should be investigated to guide clinical practice.</p>