Curriculum materials explicitly designed to foster teacher learning represent a prominent route to professional development (PD) for teachers. However, it is unclear whether PD can be delivered successfully in the form of teacher self-study of curriculum materials, or whether it has to be scaffolded additionally by an expert. This study investigated effects of expert scaffolding in science-related PD for elementary school teachers with regard to proximal teacher outcomes (i.e., teacher beliefs and motivations), instructional quality, and student achievement. Moreover, mediation of PD effects through proximal teacher outcomes and teachers' instruction was examined. Seventy-three teachers and 1,039 students participated in the study. Expert scaffolding was implemented in a 3-tiered way: A first group (18 teachers) received PD with extensive scaffolding, a second group (18 teachers) was provided with reduced expert scaffolding, and a third group (18 teachers) received no expert scaffolding and was provided with the curriculum materials only (self-study group). A baseline group (19 teachers) did not participate in science-related PD and completed questionnaires on teacher outcomes only. Scaffolded PD was significantly superior to PD through self-study in terms of teacher beliefs and motivation, instructional quality, and student achievement. Contrary to our hypothesis, PD effects on student learning were mediated only to a small extent by teacher beliefs. However, teachers' instruction emerged as a substantial mediator of PD effects on student achievement. The results highlight the advantages of additional expert scaffolding in PD based on curriculum materials for the preparation of elementary school teachers for teaching science.