While regions like the primary motor cortex (M1), left dorsolateral prefrontal cortex (DLPFC), and cerebellum are linked to cognitive functions, it is unclear which offers the greatest cognitive benefit. This study aimed to assess the effects of high-definition transcranial direct current stimulation (HD-tDCS) on these regions, focusing on inhibitory control, cognitive flexibility, and working memory. This parallel, randomized, double-blinded, and controlled trial involved 80 older adults, randomly assigned to one of four groups: anodal stimulation of M1, left DLPFC, cerebellum, or sham. Inhibitory control was assessed using reaction time (RT) and rate-correct score (RCS) from a Go/No-Go task. Trail Making Test A (TMT-A) and B (TMT-B) measured processing speed and cognitive flexibility, while the backward digit span test evaluated working memory. All assessments were conducted pre- and post-stimulation. Notably, cerebellum stimulation significantly improved working memory (p = 0.010), whereas M1, DLPFC, and sham did not. Significant interaction effects emerged for TMT-A and TMT-B, with both M1 and DLPFC stimulation enhancing performance (TMT-A: p = 0.005, p = 0.025; TMT-B: p < 0.001, p = 0.045, respectively), while cerebellum and sham had no significant impact. Additionally, RT and RCS showed no significant effects. Anodal stimulation of M1 and DLPFC improved cognitive flexibility and processing speed, whereas cerebellum stimulation selectively enhanced working memory. However, inhibitory control did not improve, highlighting the need for further tailored interventions. These findings underscore distinct region-specific effects of tDCS on cognitive performance in older adults.