Background: Depressive disorder in adolescents is a clinically heterogeneous condition with poorly defined molecular underpinnings. Glycosylation, a key post-translational modification involved in immune and neuronal regulation, has not been systematically studied across depressive subtypes in youth. Methods: We employed liquid chromatography-tandem mass spectrometry (LC-MS/MS) combined with site-specific glycoproteomic profiling to analyze serum samples from adolescents diagnosed with major depressive disorder, stratified into three subtypes: depression, depression with non-suicidal self-injury (NSSI), and depression with suicide attempts. Healthy adolescents served as controls. Intact N-glycopeptides were quantified to identify subtype-specific glycosylation patterns. Results: Distinct alterations in serum N-glycosylation were observed across depressive subtypes. All groups exhibited decreased levels of bi-antennary glycans and LacNAc/Lewis structures, alongside elevated expression of tetra-antennary sialylated glycans, particularly N6H7S4. NSSI cases showed increased tri-antennary sialylation, while suicidal depression was marked by specific upregulation of N6H7S4 on ORM2. Differentially glycosylated proteins, including SERPING1 and A2M, were enriched in immune, complement, and coagulation pathways. These changes occurred independently of total protein abundance, indicating glycan-specific dysregulation. Conclusion: Our findings uncover subtype-dependent N-glycosylation signatures in adolescent depression and implicate glycan-mediated immune and inflammatory mechanisms in disease heterogeneity. Glycoproteomic profiling offers new insights into biomarker discovery and the molecular classification of depressive subtypes.