Objective: The current longitudinal impact of treatment of ADHD on intimate partner violence (ITAP) study aims to investigate whether decrease of ADHD symptoms is associated with reduction of intimate partner violence (IPV) frequency in IPV offenders with ADHD in a forensic psychiatry setting. Method: Of n = 209 offenders of IPV with ADHD, frequency of IPV and ADHD symptoms were assessed at the 8th, 16th, 24th, and 52nd weeks of their combined treatment for ADHD and IPV. Results: We observed a significant decrease of self-reported ADHD symptoms (large effect size, d ≥ 0.80) and all IPV outcomes (small, d > 0.20, to large, d > 0.80, effect sizes). The decrease in IPV was mainly associated with the decrease in ADHD symptoms. Conclusion: As IPV treatment alone is not effective in the reduction of IPV in forensic psychiatry, we now have strong indications that the combined treatment of adult ADHD and IPV is more effective in offenders with ADHD.