<p>The current study aimed to investigate the effectiveness of transcranial direct current stimulation (tDCS) in improving the clinical symptoms and cognitive functions in adults diagnosed with attention deficit/hyperactivity disorder (ADHD) as well as its treatment acceptability. Randomized placebo-controlled trials (RCTs) identified from major databases using the main keywords “tDCS” and “ADHD” were reviewed for improvement in clinical symptoms and cognitive functions in adults diagnosed with ADHD. The outcomes of interest were quantified using effect size expressed as standardized mean difference (SMD) and odds ratios (ORs) for continuous and categorical variables, respectively, with 95% confidence interval (CI). Meta-analysis of seven RCTs including 263 participants (mean age = 31.37) revealed significantly a greater improvement in the symptoms of inattention (SMD=-1.24, 95% CI:-0.84 to -1.63, <i>p</i> &lt; 0.01) and cognitive tasks involving attentional performance (SMD= -0.54, 95% CI: -1.07 to -0.01, <i>p</i> = 0.04) in the tDCS group compared with the control group, but no significant difference in the symptoms of hyperactivity/impulsivity (<i>p</i> = 0.19) and inhibitory control (<i>p</i> = 0.17) between the two groups. There was also no significant difference in dropout rate between adults receiving tDCS and their controls (<i>p</i> = 0.07). Our study supported the use of tDCS for improving the self-reported symptoms of inattention and objective attentional performance in adults diagnosed with ADHD. However, the limited number of available trials hindered a robust investigation into the parameters required for establishing a standard protocol such as the optimal location of electrode placement and treatment frequency in this setting. Further large-scale studies are warranted to address these issues.</p>