ObjectiveThe ‘Ringing Up about Breastfeeding earlY’ (RUBY) randomised controlled trial showed increased breastfeeding at 6 months in participants who received the proactive telephone-based peer support breastfeeding intervention compared with participants allocated to receive standard care and supports. The present study aimed to evaluate if the intervention was cost-effective.DesignA within-trial cost-effectiveness analysis.SettingThree metropolitan maternity services in Melbourne, Victoria, Australia.ParticipantsFirst time mothers intending to breastfeed their infant (1152) and peer volunteers (246).InterventionThe intervention comprised proactive telephone-based support from a peer volunteer from early postpartum up to 6 months. Participants were allocated to usual care (n=578) or the intervention (n=574).Main outcome measuresCosts during a 6-month follow-up period including individual healthcare, breastfeeding support and intervention costs in all participants, and an incremental cost-effectiveness ratio.ResultsCosts per mother supported were valued at $263.75 (or $90.33 excluding costs of donated volunteer time). There was no difference between the two arms in costs for infant and mothers in healthcare and breastfeeding support costs. These figures result in an incremental cost-effectiveness ratio of $4146 ($1393 if volunteer time excluded) per additional mother breast feeding at 6 months.ConclusionConsidering the significant improvement in breastfeeding outcomes, this intervention is potentially cost-effective. These findings, along with the high value placed on the intervention by women and peer volunteers provides robust evidence to upscale the implementation of this intervention.Trial registration numberACTRN12612001024831.