There is a paucity of interventions for young adults who have parents with a mental health or substance use issue. The 6-week mi.spot (supportive, preventive, online, and targeted) professionally moderated, online intervention fills this gap. The purpose of this study was to present evidence of the acceptability, safety, and preliminary effectiveness of this intervention.In response to social media advertising, 31 young people aged 18-25 years participated in mi.spot. Intervention effectiveness was examined via a single-group pre, post, and 6-week follow-up study design, measuring primary changes in depression, anxiety, stress, and psychological well-being and secondary changes in coping, self-efficacy, social connectedness, attribution of responsibility, help-seeking, and mental health literacy. Acceptability and safety were determined by system use and participants' self-reports.Over the 6 weeks of the intervention, 28 (90.3%) of 31 participants used one or more components of the mi.spot intervention. Significant improvements were reported in depression and stress from preintervention to 6-week postintervention. Trend improvements were evident in well-being, social connection, and coping. No change was reported in general help-seeking, social connectedness, mental health literacy, self-efficacy, or attribution. No safety violations were reported. Participants reported mi.spot to be safe and acceptable.mi.spot appears to be safe and acceptable and shows promise as an effective online intervention to improve the mental health and well-being of young adults with parents with mental health and/or substance use issues.