Though autism spectrum disorder (ASD) traits are associated with depression, it is unclear if school social capital mediates their association. We examined whether school social capital mediates the association between ASD traits and depression, and moderation effect of sex on the mediation effect among adolescents in a general population sample (1750 males, 1779 females; equivalent 12–15 years old). The results of this study indicate that ASD traits are associated with depression among adolescents, and that this association is partly mediated by school social capital. Furthermore, the results of the moderated mediation analysis suggest that lower level of school social capital can lead to more increase level of depression for females than for males.