Background: Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder of childhood which manifests as symptoms of inattention, hyperactivity, and impulsivity. Although typically considered a condition affecting only children, it persists into adulthood leading to significant impairments and poor quality of life and meaningful existence. Aims: To date, there is a scarcity of research that investigated ADHD in adults in nonwestern populations and Oman is no exception. This study aims to estimate the observed and standardized five-year hospital-prevalence estimates of ADHD among adults in a tertiary hospital in Oman. Methods: This was a hospital-based cross-sectional study, utilizing reports of adult ADHD cases during the 5-year study period of a tertiary hospital. Inclusion and exclusion criteria were applied to participants and several variables were extracted for the included cases. Multiple data analysis techniques were applied to the data of this study, including descriptive statistics, chi-square tests and prevalent estimates calculation procedures. In addition, indirect standardization was employed to aid in calculating the standardized governorate-specific prevalence estimates of adult ADHD. Results: Out of 39,881 visits reported to the Behavioral Medicine department of the present tertiary hospital, adults with a diagnosis of ADHD accounted for 1.77% of all the visits. The overall prevalence estimate of visits in this study was 17.8 (CI: 16.5-19.1) per 1000 outpatient visits (OPD). The highest prevalence was reported in the age group that is less than 20 years old (104.7 per 1000 OPD). Males had higher prevalent rates than their females’ counterparts. In terms of governorate-specific prevalence, Dhofar exhibited the highest observed and standardized prevalence rates with 36.9 and 52.0 per 1,000 OPD respectively. The inattentive subtype of ADHD was the most common, accounting for 63.5% of cases. Substance use disorder (SUD) and anxiety were the most common comorbidities reported in 32.4% & 22% of the participants respectively. 42.3% of the participants were either overweight or obese. Methylphenidate was the most prescribed medicine, given to 76.1% of the study participants. 2021 exhibited the highest adult ADHD prevalence (21.7, CI: 21.7-25.1) whereas 2020 had the lowest observed prevalence (13.3, CI: 13.3-16.5) out of all five years of the study period. Conclusions: To date, this is the first study to determine the prevalence of adult ADHD seeking consultation from 2018-2022. The study explored the demographic characteristics and subtype of ADHD, assessed comorbidity and medical history among adults with ADHD, analyzed the medications prescribed to adult ADHD patients, and determined the prevalence of adult ADHD in each governorate of Oman. Overall, adult ADHD was more prevalent in males than females with younger males group reporting the highest prevalence in the study. Obesity & overweight were common among adults with ADHD. The inattentive subtype being the most prominent among participants. SUD and anxiety were the most common comorbidities. Dhofar Governorate having highest prevalence in the sultanate of Oman and the most prescribed medication was Methylphenidate. 8 This research project’s findings should be considered a stepping-stone for future studies to build upon, as it is only the 2nd study in Oman to focus on adult ADHD, and the 1st of its kind to investigate adult ADHD prevalence. This research found similarities in many aspects and dimensions of adult ADHD with previously published literature, which leads to the thought that the magnitude and burden in Oman are not less serious than other countries. It also indicates that it is a potential health hazard that should be carefully recognized, investigated, and dealt with.