Background <p>Impulsivity in individuals with attention-deficit/hyperactivity disorder (ADHD) diminishes quality of life. Cortical hypometabolism has been hypothesized to contribute to ADHD. Transcranial photobiomodulation (tPBM) is a safe, non-invasive method for stimulating the prefrontal cortex (PFC), yet its cognition-enhancing effect on impulsivity and sustained attention in adults with ADHD remains understudied. This research evaluated whether acute tPBM administration could improve impulsivity and prefrontal oxygenation.</p> Methods <p>The current investigation was conducted in two stages. The first stage included 223 participants with and without a history of ADHD, whereas the second stage focused exclusively on 52 adults with ADHD. In the first stage, participants were randomized to receive either active or sham 1064&#xa0;nm laser tPBM applied to the right PFC, followed by completion of the continuous performance task (CPT). This stage examined the behavioral effects of tPBM on impulsivity and attentional control in adults with ADHD compared with non-ADHD participants. In the second stage, participants completed the CPT before and after the tPBM session. In addition, functional near-infrared spectroscopy (fNIRS) data were acquired before and after tPBM while participants performed a 2-back working memory task, enabling assessment of prefrontal hemodynamic responses.</p> Results <p>Behavioral results from the first stage showed that, within the ADHD group, participants receiving active tPBM exhibited a 51% improvement in correct rejections on the CPT compared to sham, reaching levels comparable to non-ADHD participants (<i>p</i> = 0.017, large effect size of Cohen’s <i>d</i> = 0.93). In the second stage, exploratory analyses suggested that participants receiving ADHD medication showed greater improvements following tPBM compared to non-medicated individuals (<i>p</i> = 0.049), which should be interpreted cautiously. Active tPBM also improved correct rejections in the 2-back task (<i>p</i> = 0.040). fNIRS analyses demonstrated increased prefrontal oxygenation following active tPBM, with higher oxyhemoglobin (<i>p</i> = 0.011) and total hemoglobin (<i>p</i> = 0.038) compared to sham. No adverse events were reported. No sex differences were found in the tPBM effects.</p> Conclusions <p>These findings suggest that enhancing prefrontal oxygenation via tPBM may strengthen neural networks supporting cognitive functions in adults with ADHD and merit evaluation in repeated-session studies.</p> Trial registration <p>ClinicalTrials.gov (ID: NCT05802680) on 03-20-2023 (retrospectively registered). <a href="https://clinicaltrials.gov/study/NCT05802680?term=NCT05802680%26rank=1">https://clinicaltrials.gov/study/NCT05802680?term=NCT05802680&amp;rank=1</a>.</p>