Research has shown that some students who underperform in mathematics overestimate their performance, while others who excel in mathematics underestimate it. Looking at this mismatch of performance and confidence judgement--the Dunning-Kruger effect (DKE)--the current study investigates how well students' confidence judgement and item-specific mathematics competence relate with each other and whether such a relationship differs across six European countries (i.e., Norway, Sweden, Finland, Estonia, Serbia and Portugal). We also examine whether perceived competence, mathematics identity, gender, socioeconomic status and immigration background predict this mismatch and whether these demographic factors function differently between the examined countries. The results show that the DKE could be found across grades three and four in all six countries. However, there are country-specific patterns regarding the relationship between performance, mathematics identity and perceived competence; the DKE; and how different demographic variables predict its occurrences in particular subpopulations.