Introduction: Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental condition that affects functioning across the lifespan. Functional Near-Infrared Spectroscopy (fNIRS) as non-invasive brain imaging technique that uses light to measure brain activity can be used to investigate neural mechanisms in ADHD. Voluntary attention is frequently impaired in ADHD, making it important to assess bilateral activity in frontoparietal networks to identify potential discrepancies. Purpose: Investigate differences in oxygenated hemoglobin (HbO) concentration, measured with fNIRS, in the left and right frontal and parietal cortices during a voluntary attention task, comparing groups with and without clinical signs of ADHD. Method: Forty-eight right-handed Brazilian young adults (Mean Age = 22.7, SD = 6.77) participated in the study. ADHD symptoms were assessed using the Adult ADHD Self-Report Scale (ASRS-18), that classified participants into Control (n = 18) and Clinical (n = 30) groups using a 21-point cutoff. Participants completed a cue-target paradigm with centrally presented predictive cues under valid and invalid conditions in a block-design. Brain activation was recorded from 26 channels covering bilateral frontal and parietal cortices. Group comparisons were made using the Mann-Whitney test. Results: The Clinical group showed significantly lower HbO activity (Mean = -0.01167) in the right prefrontal cortex (U = 165, p = 0.025, r = -0.39) compared to the Control group (Mean = 0.04045), suggesting reduced recruitment of this region, particularly in the right hemisphere, during voluntary attention. No significant differences were observed in the other monitored regions. Conclusions: The observed pattern of right prefrontal hypoactivity supports previous findings that link ADHD to deficits in inhibitory control, voluntary attention, and executive functions. These results suggest that right frontal activity could serve as a potential neural marker for ADHD, highlighting the value of fNIRS as a reliable tool for investigating and monitoring neurofunctional differences associated with clinical symptoms.