Teacher self-efficacy plays a key role in the successful inclusion of children with intellectual disabilities in mainstream schools. But what influences self-efficacy and how can we support its growth? Drawing on Social Cognitive Theory, the study examined teachers’ mastery experiences, perceptions of the school environment, self-efficacy and reported inclusive teaching. The sample comprised 148 primary school teachers from Scottish mainstream schools. Participants completed questionnaires measuring mastery experiences, the school environment (collective efficacy and school climate perceptions), self-efficacy and reported inclusive teaching practices. Regression analyses demonstrated that school environment (collective efficacy and school climate), and mastery experiences were important in predicting teachers’ self-efficacy. Further, self-efficacy acted as a mediator between teachers’ perceptions of the school climate and reported inclusive behaviour. This brings us closer to understanding how teacher self-efficacy is fostered and the role of the school environment. Engaging with teacher belief systems may cultivate a school climate that promotes inclusion.