Background: National and international guidelines advocate for a multimodal approach to treating adult attention deficit hyperactivity disorder (ADHD), combining pharmacotherapy with psychological interventions. While recent reviews support cognitive behavior therapy (CBT) as a viable treatment for ADHD in adults, evidence remains limited. Another challenge is the availability of such interventions, with stimulants remaining the primary treatment choice for ADHD adults. One promising approach to increasing access to psychological interventions is the dissemination of internet-delivered CBT (iCBT). Objective: This study evaluated the #feasibility, acceptability, and effects of a guided internet-based stress-management program specifically designed for working adults with ADHD. The intervention aimed to enhance quality of life by addressing stress, exhaustion, anxiety, and depression, commonly experienced by this population. Methods: Thirty-six participants took part in a single-arm open trial, with assessments before, during, and after the intervention. The intervention consisted of 12 modules based on CBT principles, focusing on executive functioning, stress management, and emotion regulation, with clinical support on demand. Results: Results indicated a statistically and clinically significant improvement in quality of life, and a reduction in ADHD symptoms, as well as statistically significant reductions in perceived stress, exhaustion, anxiety, and depression. Participants reported high satisfaction with the program and the guidance. Adherence varied, but no explicit dropouts occurred. Twelve of the 36 participants reported negative effects. Qualitative content analysis of participants’ written feedback revealed positive experiences and suggestions for improvement. Conclusions: This study suggests promise for internet-delivered interventions tailored to the needs of ADHD adults, pending further research and development in controlled studies. Clinical Trial: https://osf.io/u2cdj