Math anxiety is a negative affective reaction in situations concerning mathematics and is related to poor math performance and a lower mathematical self-concept. Gender differences appear in math anxiety even though gender differences in math abilities are non-existent or minimal in effect size. In the present study, gender and age differences in math anxiety, mental-rotation performance, and perceived mental-rotation performance are investigated as well as (perceived) mental-rotation performance as possible mediators of the relation between gender and math anxiety. Ninety-seven children (54 females) between 11 and 15 years and 84 undergraduate students (59 females) solved a mental-rotation test, rated their performance in this test, and filled out a questionnaire about math anxiety. Increasing gender differences with age were found for math anxiety, mental-rotation performance, and perceived mental-rotation performance. Mental-rotation performance and perceived mental-rotation performance were significant individual mediators for gender differences in math anxiety. Adequate measures should be discussed to enhance females' mental-rotation abilities and their perception of these skills to reduce gender differences in math anxiety.