Guiding students' STEM problem solving entails dynamic processes driven by changes in real-world contexts. To understand these processes, we aimed to identify the formation and influence of 'norms' as shared behaviour patterns desirable in STEM problem-solving. To this end, 10 sessions of STEM lessons for secondary students were carried out, and we collected data comprising lesson observation videos, fieldnotes, interviews, and photographs of students' artifacts. The data were analysed based on three features of norms: justifiability, sharing, and behaviours. The results report three epistemic norms in STEM problem-solving. First, the norm of 'defining a real-world problem that is useful and accessible' was found. While the students searched for useful problems in their everyday lives, they also needed to ensure that these problems were scoped such that they had the capacity to manage them. The second norm was 'designing creative and concrete prototypes'. Students' prototypes were expected to be creative in addressing the established problems and to be developed in a concrete manner. The last norm identified was 'testing and revising in more iterative and feasible ways'. The students tested their models repeatedly in a manner aligning with their skills and the materials provided. Based on the findings, educational implications are discussed in terms of understanding and facilitating STEM problem-solving.