Abstract Background: Breastfeeding is a major public health concern and a decision to breastfeed has important health benefits for both the infant and mother. Breastfeeding rates are low in many Western countries, and evidence is accumulating to suggest that breastfeeding should be promoted in schools. The present study aimed to employ the theory of planned behaviour (TPB) to predict and explain young people’s motivations to breastfeed with a view to designing an intervention to positively promote breastfeeding among adolescents. Methods: Senior schools in Northern Ireland were sorted by a number of key stratification variables, and then randomly sampled. In anticipation of a nonresponse, a first and second reserve list was drawn using matched replacements. Selected schools were contacted by letter, followed by a telephone call. Thirty-six schools agreed to take part in the research, and a cross-sectional survey was conducted with 2021 Year 10 pupils. Results: Having been breastfed as a child and having seen a mother breastfeeding significantly correlated with intention among both females and males. In descending order of importance, the TPB direct constructs were all significantly correlated with intention among males and females. A hierarchical multiple regression was performed with the intention to breastfeed/encourage partner to breastfeed as the dependent variable, and the TPB successfully predicted intention among males and females. Conclusions: The findings obtained in the present study suggest that the TPB is a useful framework on which to base the design of an intervention to promote attitude change, and highlight the importance of key theoretical constructs in predicting intentions to breastfeed.