This paper reports a pretest-posttest study about the impact of a teacher professional development (TPD) programme on primary school teachers' knowledge of and attitude towards inquiry-based learning. A pedagogical framework of inquiry phases and domains of scientific knowledge combined with hard and soft scaffolds formed the basis for the TPD programme. A total of 59 teachers were divided between the experimental group, which participated in the TPD programme, and a control group. We measured the teachers' subject matter knowledge (SMK) of the conceptual, epistemic, social, and procedural domain before and after the TPD programme by means of different questionnaires. In addition, we measured their knowledge of how to support their pupils during the inquiry process (PCK) and their attitude towards inquiry-based learning. The results show that our TPD programme improved teachers' conceptual and social SMK, PCK, and attitude. Our study implicates that scaffolding different domains of scientific knowledge during the inquiry cycle is a valuable component of TPD in inquiry-based learning.