Integrated STEM education has been the focus of recent curriculum reforms worldwide. Learning through design is a distinctive and significant approach to achieving STEM integration. Although studies have investigated various outcomes of design-based STEM learning, little has been done to examine its influence on two significant learning outcomes--STEM creativity and epistemic beliefs, especially in the context of primary education. In this study, we investigated the effects of a 6-month design-based STEM learning event (in which the application of information technologies was required) on 155 upper primary students' STEM creativity and epistemic beliefs. The results showed (i) a significant improvement in the fluency and flexibility dimensions of STEM creativity, (ii) a significant decrease in the source, certainty, and justification dimensions of epistemic beliefs, and (iii) no statistically significant change in the originality dimension of STEM creativity or the complexity dimension of epistemic beliefs. Moreover, we found significant and negative correlations between STEM creativity and the source, certainty, and justification dimensions of epistemic beliefs, but a significant and positive correlation between STEM creativity and the complexity dimension of epistemic beliefs. The findings of this study generate additional evidence to support design-based learning as a meaningful and feasible approach to achieving STEM integration in primary schools. Suggestions are made on how to design and implement integrated STEM education in primary schools.