The primary objective of this study was to evaluate the efficacy of a continuing professional development (CPD) intervention in producing changes in physical education (PE) teaching practice and PE teaching quality by generalist primary school teachers when the CPD addressed the use of a game-centred approach. A cluster randomized controlled trial was conducted in seven primary schools in the Hunter Region, New South Wales, Australia. One year six teacher from each school was randomized into the Professional Learning for Understanding Games Education (PLUNGE) intervention (n = 4 teachers) or the 7-week wait-list control (n = 3) condition. The PLUNGE intervention (weeks 1–5) used an instructional framework to improve teachers’ knowledge, understanding and delivery of a game-centred curriculum, and included an information session and weekly in-class mentoring. The intervention was designed to enhance content and pedagogical knowledge for the provision of pedagogy focused on a broad range of learning outcomes. Teaching quality was assessed at baseline and follow-up (weeks 6 and 7) via observation of two consecutive PE lessons using the Quality Teaching Lesson Observation Scales. Linear mixed models revealed significant group-by-time intervention effects (p < 0.05) for the quality of teaching (effect size: d = 1.7). CPD using an information session and mentoring, and a focus on the development of the quality of teaching using a game-centred pedagogical approach was efficacious in improving the quality of PE teaching among generalist primary school teachers.