The aim of the present study was to examine, for the first time, the level of gender-stereotyped beliefs about STEM-related school subjects among Croatian primary school students and to explore how stereotyped beliefs can be predicted from prior achievement in STEM school subjects and students' STEM interests. Eight hundred and eighty primary school students (442 girls and 438 boys) completed a paper-and-pencil questionnaire in their own classrooms (data used in the study are extracted from a larger STEM research project). The measures of interest in this study were stereotype endorsement, interest in STEM-related school subjects, and school marks in these subjects. Results suggest that regardless of prior school achievement, students who have stereotype-consistent interests in school subjects tend to show stronger stereotype endorsement than others. Male gender and prior achievement in STEM-related school subjects were also positively related to stereotype endorsement. These results are discussed in light of the existing literature and some practical implications are considered.