The current study investigated the developmental trajectory of friendship jealousy across a one-year interval during adolescence. It also explored the association of attachment security with friendship jealousy while testing the potential mediating role of emotion regulation. A total of 1,158 middle school students participated in this study at three time points. A set of self-report questionnaires were adopted to assess attachment security to mother, father, and close friends, emotion regulation in general, emotion regulation for anger and sadness, and friendship jealousy. Friendship jealousy tended to be stable for girls from 7th to 8th grade. There were negative associations between parental attachment security and friendship jealousy at all three time points, but parental attachment security did not predict friendship jealousy one year later. Attachment security to close friends negatively predicted friendship jealousy concurrently and one year later. The mediating role of emotion regulation was not supported in the longitudinal analyses.