While it is well established that positive peer relationships lead to positive outcomes, little is known about the mediating role of school motivation on the association between perceived peer support and school achievement among middle grades students. Even less is known about the effect of gender on this mediation. The current study examined whether school motivation mediates the association between perceived peer support and achievement. In addition, the gender effect on the mediation was examined using multigroup structural equation modeling. A total of 501 middle grades students (259 girls and 242 boys, aged 10-14 years) completed the self-report questionnaires assessing their perceived peer support, school motivation, and achievement. All models showed good model-data fit and indicated that school motivation fully mediates the associations between perceived peer support and school achievement in both genders. Also, the findings suggested that the association between school motivation and achievement was moderated by gender. The findings were discussed, and implications were shared for enhancing school achievement of middle grade students.