Purpose: Damage to the uterosacral ligaments is an important contributor to uterine and vaginal prolapse. The aim of this study was to identify differentially expressed proteins in the uterosacral ligaments of women with and without pelvic organ prolapse and analyze their relationships to cellular mechanisms involved in the pathogenesis of pelvic organ prolapse. Experimental Design: Uterosacral ligament connective tissue from four patients with pelvic organ prolapse and four control women underwent iTRAQ analysis followed by Ingenuity Pathway Analysis of differentially expressed proteins. Differentially expressed proteins were valideated using western blot analysis. Results: A total of 1789 unique protein sequences were identified in the uterosacral ligament connective tissues. 88 proteins had expression levels that were significantly different between prolapse and control groups (≥1.2-fold). Ingenuity pathway analysis demonstrated 14 differentially expressed proteins that were associated with "Connective Tissue Function". Among them, fibromodulin(FMOD), Collagen alpha-1 (XIV) chain(COL14A1), Calponin-1 (CNN-1), Tenascin (TNC), and Galectin-1(LGALS1 appeared most likely to play a role in the etiology of pelvic organ prolapse. Conclusions and clinical relevence: We identified at least 6 proteins not previously associated with the pathogenesis of pelvic organ prolapse with biologic functions that suggest a plausible relationship to the disorder. These results may be helpful for furthering our understanding of the pathophysiological mechanisms of pelvic organ prolapse.