Currently, Attention-deficit/Hyperactivity disorder (ADHD) is diagnosed by a clinical physician by going through a physical examination and a series of interview with the patient and significant people around them based on the DSM-5. This is time consuming as it typically takes up to 3 h and it is not always objective. Our research investigates the possibility of utilizing a portable functional Near-Infrared Spectroscopy (fNIRS) device known as the NIRSIT-Lite as an objective and convenient tool for detecting ADHD in adolescents. The portable fNIRS device measures oxygenated and deoxygenated haemoglobin concentrations (HbO2 levels), utilizing signal processing to discern neurophysiological patterns found in the prefrontal cortexes of the adolescents. Through a Go/No-go (GNG) paradigm and Verbal Fluency Task (VFT), our study focuses on real-time responses in the prefrontal cortex during cognitive tasks. Comparative analyses between ADHD and control groups aim to establish the device's sensitivity and specificity and assess its ability to be used to potentially identify individuals with ADHD. Our results show that it is indeed true (as shown by the overall p < 0.05) and that using NIRSIT-Lite while performing the Go/No-go task and Verbal Fluency Task can be an effective and accurate method to diagnose ADHD in adolescents. Our results also show that high changes in HbO2 levels indicate healthy adolescents while low changes indicate that an adolescent may be diagnosed with ADHD. These results could revolutionize ADHD diagnostics by providing a non-invasive, portable, fast, and objective method for early detection.