The aim of this research was to evaluate the effectiveness of a school-based intervention designed to enhance young peoples motivations to breastfeed. A cluster randomized controlled trial was conducted involving 50 post-primary schools from across Northern Ireland. However, dropout and exclusion criteria utilized for the current study resulted in an effective sample size of 42 schools. The 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 the TPB were administered to all intervention and control schools at baseline, 1 and 6 months post-intervention. Multi-level modelling was used to analyse the data. Findings 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. The research has provided evidence to support the use of the TPB in the design and evaluation of an intervention to increase females intentions to breastfeed.