Background <p>Both attention-deficit hyperactivity disorder (ADHD) and post-traumatic stress disorder (PTSD) are complicated illnesses that sometimes co-occur in children and adults with significant negative influence on a person’s life and general well-being.</p> Aim <p>This study aims to conduct a systematic review that investigates the comorbidity of PTSD and ADHD in the adult population.</p> Methods <p>A comprehensive search was conducted across five electronic databases (PsycNET, Cochrane, PubMed, Google Scholar, and ClinicalTrials.gov) between October 5 and 20, 2023, using predefined keywords including “ADHD,” “PTSD,” and “comorbidity.” Studies were included if they involved adult participants (≥ 18 years) with both ADHD and PTSD diagnoses. Two independent reviewers conducted screening and data extraction. No meta-analysis was performed due to heterogeneity in study designs. The results were synthesized qualitatively.</p> Results <p>Out of 818 identified studies, 21 met the inclusion criteria. Studies reported an increased risk of developing PTSD in individuals with ADHD, with the prevalence of comorbidity ranging between 28 and 36%. ADHD in PTSD patients was associated with greater psychosocial impairment, more severe PTSD symptoms, and functional difficulties. Treatment approaches, including pharmacotherapy (atomoxetine, Vyvanse) and mindfulness training, showed potential benefits for managing comorbid symptoms.</p> Conclusions <p>ADHD and PTSD commonly co-occur in adults, leading to more severe clinical outcomes. Early diagnosis and effective treatment of ADHD may reduce the risk of developing PTSD. Further research is needed to explore the underlying mechanisms and optimal treatment strategies for individuals with ADHD/PTSD comorbidity.</p> Systematic review registration <p>PROSPERO CRD42023479631.</p>