Background: Further developing students' thinking about knowledge and knowing in science (epistemic beliefs) is considered a normative goal of science education in many countries around the world, even for elementary-school-aged children. Aims: The goal of the present study was to introduce and evaluate a new intervention in science education aimed at developing children's epistemic beliefs, epistemic curiosity, and investigative interests. The intervention included an inquiry-based learning approach as well as reflections on epistemic issues because these methods are currently seen as most promising for fostering students' epistemic beliefs. Sample: Data were collected from 65 elementary school children in Grades 3 and 4 (58.46% boys, age: M = 8.73, SD = 0.60) who participated in a voluntary extracurricular STEM enrichment programme in south-west Germany. Methods: We investigated the effectiveness of the intervention by applying a randomized block design with a treated control group and repeated measures. The effectiveness of the intervention was analysed via multiple linear regression analyses. Results: The results indicated that the children assigned to the intervention developed more sophisticated epistemic beliefs and a higher level of epistemic curiosity than the children assigned to the control condition. No intervention effects were found on investigative interests. Conclusions: The results provide initial evidence for the effectiveness of the intervention and demonstrate that it is possible to improve epistemic beliefs among elementary school children in Grades 3 and 4. The study provides a starting point for understanding how young children develop epistemic beliefs.