A growing body of scholarship in the field of writing research from a cognitive perspective suggests that girls tend to outperform boys in particular writing tasks. Still, our understanding about gender differences continues to evolve. The present study specifically focused on gender differences in writing between students from Grade 4 to Grade 9. We examined differences in handwriting and self-efficacy, as well as in three measures of written composition across two genres (viz., spelling, text length, and text quality in stories and opinion essays). Moreover, we tested whether there were differences in written composition above and beyond handwriting and self-efficacy. Findings suggest that girls consistently outperformed boys in handwriting, self-efficacy, spelling, text length, and text quality. These effects were moderated by neither students’ grade nor text genre. In addition, after accounting for handwriting and self-efficacy, females still performed better than males in the three measures of written composition. Overall, findings confirmed the gender difference typically found in writing and indicated that potential explanatory variables for it may be handwriting and self-efficacy.