Endometriosis (EM) is a chronic inflammatory disease characterized by the growth of endometrial-like tissue outside the uterus, yet its molecular mechanisms remain poorly understood. This study investigated the expression of mucins (MUC1, MUC2, MUC5AC, MUC6, MUC16) and their O-glycans in endometriotic lesions, given their roles in epithelial protection, adhesion, and immune modulation. Using immunohistochemistry, Western blotting, lectin profiling, histochemical staining, and transcriptomic analysis, we compared mucin levels and glycosylation patterns in eutopic and ectopic tissues from women with and without endometriosis and measured mucin-derived tumor markers in serum (CA 125/MUC16 and CA 15-3/MUC1) and peritoneal fluid (CA 125/MUC16). The results showed significant upregulation of all mucins in EM biopsies, with increased MUC1 transcript levels, while MUC6 and MUC16 protein levels did not always align with transcripts. Yet, tumor markers CA 125 and CA 15-3 showed no significant differences between groups. Looking at mucin distribution in biopsies of peritoneal (pEM), deep infiltrating and ovarian EM, MUC1 was significantly overexpressed in lesions of all EM forms, while MUC5AC was significantly elevated in pEM. Lectin analysis revealed specific glycan changes, including elevated core-1 O-glycans and α(1-2)-linked fucosylation, while sialylation remained unchanged. These findings demonstrate consistent mucin dysregulation and glycan alterations, implicating their roles in epithelial adhesion, immune evasion, and lesion persistence. Mucin biology thus emerges as a promising target for diagnostic and therapeutic strategies in endometriosis.