The purpose of this study was to establish whether participation in a teacher design team (TDT) is an effective way to increase the science teaching self-efficacy of primary school teachers who vary in their levels of experience and interest in science. A TDT is a group of at least 2 teachers from the same or related subjects working together to (re)design and enact (a part of) their common curriculum (A. Handelzalts, 2009). This study considers the context of a primary school in The Netherlands. Three forms of data were collected to obtain more in-depth data about the value of working in a TDT: the Dutch version of the Science Teaching Efficacy Belief Instrument (P. Fisser, B. Ormel, & C. Velthuis, 2010), an additional TDT questionnaire, and a focus group interview. The results indicate that professional development through a TDT can improve science teaching self-efficacy for teachers with varying levels of experience and interest.