Interest in Science, Technology, Engineering and Mathematics (STEM) has gained momentum due to increasing calls for a more STEM-literate society. As teaching integrated STEM poses curricular and pedagogical challenges for most generalist primary teachers, professional development (PD) is essential to support them to develop appropriate knowledge and efficacy to teach STEM. This paper presents a qualitative study of 17 primary teachers, 2 school principals and the PD facilitator. It explores the perceived impact of a customised three-phase STEM PD program on teacher efficacy in STEM education using Bandura's (1977) sources of efficacy as an analytic lens. The findings illustrate how particular features of the PD model were identified as contributing to the development of participating teachers' performance accomplishment, emotional arousal, vicarious experiences and verbal persuasion. In particular, there was consensus that the developmental structure of the STEM PD program, in terms of teachers' assuming increasing ownership for their STEM learning, incrementally enhanced teacher efficacy in STEM education. Implications of this study for STEM education and in particular STEM PD are discussed fully.