BACKGROUND: Internet-based parental programmes may improve parental wellbeing and mitigate the burden of mental health issues during the perinatal period. However, few studies have explored the cost and clinical impacts of such interventions. In the present study, we sought to evaluate the cost-effectiveness associated with an online cognitive behaviour therapy intervention (Baby Steps Wellbeing) to an information-only programme (Baby Care). METHODS: An alongside-trial cost-effectiveness analysis was undertaken using data from a randomised clinical trial comparing the Baby Steps Wellbeing intervention to Baby Care. Direct healthcare costs, as well as indirect costs attributed to income loss, were considered. The Assessment of Quality of Life-8 Dimensions multi-attribute utility instrument was used to estimate participant utilities, and subsequently calculate quality-adjusted life years (QALYs). Incremental cost-effectiveness ratios were calculated to assess the cost-effectiveness of the intervention. The economic evaluation adopted a societal perspective. RESULTS: In total, 496 parents were randomised to either the Baby Steps Wellbeing intervention or the Baby Care control arm. No significant differences in costs (-$27, 95% confidence interval (CI): -$1189-$1134) or QALYs (0.051, 95% CI: -0.097-0.200) were identified. Bootstrapped results showed that the Baby Steps Wellbeing programme was cost-saving and health improving in 38% of simulations and cost-effective in another 37% of simulations. CONCLUSIONS: The Baby Steps Wellbeing programme was slightly cost-saving with slightly improved health outcomes compared with Baby Care. Bootstrapped results indicate the Baby Steps Wellbeing was cost-effective in 75% of simulations. Overall, the Baby Steps Wellbeing programme is an online programme that is cost-effective. TRIAL REGISTRATION: Australian & New Zealand Clinical Trials Registry: ANZCTR12614001256662.