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Functional near-infrared spectroscopy (fNIRS) is used as a non-invasive neuroimaging technique for studying the neural dynamics within the human brain. This study uses fNIRS to explore how transcranial alternating current stimulation (tACS) affects cognitive function in people with dementia including Alzheimer’s disease (AD). By measuring changes in blood oxygenation levels, fNIRS provides a valuable means to capture plausible post-treatment alterations in cerebral hemodynamics. Previous studies have shown individuals with dementia exhibit increased oxygen consumption during verbal fluency tasks. This suggested that they might require greater effort to compensate for cognitive deficits. Therefore, it is hypothesized that tACS intervention will potentially alleviate this increased oxygen consumption by enhancing cognitive function, resulting in reduced oxyhemoglobin level post-treatment amongst those who respond positively to tACS treatment. To investigate this hypothesis, this research involved a comprehensive examination of 29 individuals with dementia, who received both real (40 Hz) and sham tACS treatment in a double-blind cross-over randomized trial. Each tACS intervention was paired with cognitive exercises (CE) on a daily basis in two 30-minute sessions for four consecutive weeks (excluding weekends) (tACS+CE). To reduce treatment effects between intervention, each participant had a 2-5 month wash-out period between treatments. Study participants were assigned into two groups using their age and cognitive level as the stratification parameters. One group received real tACS first and sham second (R1S2), and the other group’s real/sham assignment was the opposite (S1R2). An eight-channel fNIRS device was utilized to measure blood oxygenation levels in the prefrontal cortex (PFC) before and after each treatment block, during a mental task (word production and subtraction tasks). The results of this work are in support of our hypothesis that patients with dementia