BACKGROUND:Little empirical evidence is available to support the effectiveness and cost-effectiveness of internet interventions to increase help-seeking behavior for mental health in young adults. OBJECTIVE:The aim of this study was to evaluate the cost-effectiveness of a Web-based mental health help-seeking navigation tool (Link) in comparison with usual help-seeking strategies. METHODS:A cost-utility analysis alongside the main randomized trial of Link was conducted from the Australian health care sector perspective. Young adults aged 18 to 25 years were randomized to the Link intervention (n=205) or usual care (n=208) with 1- and 3-month follow-ups. The primary outcome of this study was quality-adjusted life years (QALYs) measured by the assessment of quality of life-4D. Costs were calculated based on the self-reported resource use questionnaire and were reported in 2015 Australian dollars. Primary analyses were conducted as intention-to-treat and reported as incremental cost-effectiveness ratios. Completer analyses were conducted in a sensitivity analysis. RESULTS:Significantly more QALYs were gained in the intervention group than the control group (0.15 vs 0.14; P<.001). The intervention was associated with significantly lower health professional consultation costs at 1-month follow-up (mean costs Aus $98 vs Aus $162; P<.05). Costs of hospital services were lower at 3 months in the intervention arm (mean costs Aus $47 vs Aus $101); however, there was insufficient sample size to detect a significant difference between the groups. There were no statistically significant differences in the total costs between the 2 arms. Relative to the control group, those who received the intervention experienced 0.01 more QALYs (0.00-0.02) and had lower total health sector costs of Aus -$81 (Aus -$348 to Aus $186) over 3 months. The intervention was found to be more effective and less costly compared with usual help-seeking strategies. The intervention was 100% likely to be cost-effective below a willingness-to-pay value-for-money threshold of Aus $28,033 per QALY. Results were robust in the sensitivity analysis. CONCLUSIONS:Our study found that the online youth mental health help-seeking Web service is a cost-effective intervention for young people aged 18 to 25 years compared with usual search strategies. Further research is required to confirm these results. TRIAL REGISTRATION:Australian New Zealand Clinical Trials Registry ACTRN12614001223628; https://www.anzctr.org.au /Trial/Registration/TrialReview.aspx?id=366731.