ObjectiveThe aim of this research was to evaluate the effectiveness of a school‐based intervention designed to enhance young people's motivations to breastfeed.DesignA cluster randomized controlled trial was conducted involving 50 post‐primary schools from acrossNorthernIreland. However, dropout and exclusion criteria utilized for the current study resulted in an effective sample size of 42 schools.MethodsThe intervention was delivered in two 35‐min classroom sessions targeting those beliefs identified by the theory of planned behaviour (TPB) as significant in predicting motivation to breastfeed. Questionnaires incorporating the key components of theTPBwere administered to all intervention and control schools at baseline, 1 and 6 months post‐intervention. Multi‐level modelling was used to analyse the data.ResultsFindings suggest that the intervention was effective in that it increased females' intentions to breastfeed, expanded their knowledge and led to more favourable attitudes and perceptions of subjective norms. Notably, females' knowledge increased more in secondary schools than in grammar schools irrespective of whether they were control or intervention schools.ConclusionThe research has provided evidence to support the use of theTPBin the design and evaluation of an intervention to increase females' intentions to breastfeed.Statement of contributionWhat is already known on this subject?The beneficial effects of breastfeeding are widely documented, and exclusive breastfeeding for the first 6 months of an infant's life continues to be encouraged. However,NorthernIreland still has the lowest rates of breastfeeding inEurope and one of the lowest rates of breastfeeding worldwide. The school system has the potential to positively promote breastfeeding, but work here is sporadic and uncoordinated. The theory of planned behaviour (TPB) is a useful framework for designing and evaluating behavioural change interventions, but as yet, no study has designed and evaluated the effectiveness of an intervention specifically targeted at those beliefs identified as important in predicting uptake of a particular behaviour.What does this study add?This study provides evidence to support: The effectiveness of a randomized controlled trial in health psychology. The efficacy of theTPBas a useful framework for the design and evaluation of a behavioural change intervention. The school system as a means of promoting positive attitudes to br