<p>Specific spatial abilities such as the Mental Rotations Test have consistently shown pronounced sex/gender differences. Previous research has investigated biological (e.g., hormones), psychological (e.g., self-beliefs about abilities), and social factors (e.g., sex/gender stereotypes) to explain these sex/gender differences. However, the combined effects of two or more of these factors have rarely been investigated. Here, we explored the complex interaction between sex/gender stereotypes and self-beliefs in spatial abilities, as well as their effects on sex/gender differences in mental rotation performance. Participants (<i>n</i> = 130; 50 males, 80 females) rated their self-beliefs in spatial abilities and subsequently completed two sex-/gender-sensitive paper-pencil mental rotation tasks that varied in task demands (adapted from Jäger &amp; Althoff, <CitationRef CitationID="CR42">1994</CitationRef> and Peters&#xa0;<CitationRef CitationID="CR63">1995</CitationRef>), following either sex/gender or neutral priming through a priming questionnaire. The results indicate that, while sex/gender differences in self-beliefs and mental rotation performance were significant, sex/gender priming did not directly influence males’ or females’ self-beliefs or mental rotation performance. Nevertheless, self-beliefs and the endorsement of sex/gender stereotypes consistently predicted females’ mental rotation performance. These findings suggest that sex/gender differences in (spatial) cognitive performance—specifically, men outperforming women—primarily emerge when women believe less in their abilities and hold stronger sex/gender stereotypes. Future research should routinely assess individuals’ beliefs in their cognitive abilities and endorsement of sex/gender stereotypes when investigating cognitive group differences (e.g., cognitive sex/gender differences), to avoid overlooking their potential role in shaping cognitive performance.</p>