Divorce is associated with a range of negative psychological consequences, including increased symptoms of anxiety, depression, and somatization. The current study presents the results of a 1-year longitudinal randomized controlled trial study of the Cooperation After Divorce online intervention platform for adults going through a divorce. Participants included 1,856 Danish divorcees who, on average, began the intervention within 1 week of legal divorce and responded to the Symptom Checklist-90-Revised anxiety, depression, and somatization subscales at baseline, 3, 6, and 12 months after juridical divorce. Data analyses consisted of linear mixed-effect model analyses, mean group comparisons, and 1-sample t tests to compare the 12-month follow-up with national normative data. The study found that the intervention platform significantly reduced anxious, depressive, and somatization symptoms among divorcees in the intervention group over a 1-year period and that the magnitude of these effects was large in effect size (Cohen's d > .78). Further, it was found that at 1 year after divorce, symptom levels of all 3 outcomes were close to the population norms for participants in the intervention group but still considerably elevated for participants in the control group. The findings suggest that online intervention platforms may be effective in reducing adverse mental health related effects of divorce and thereby offer long-term human and public health benefits. (PsycInfo Database Record (c) 2020 APA, all rights reserved).