Carbohydrate antigen 125 (CA125) is a glycoprotein commonly overexpressed in epithelial ovarian cancer and widely recognized as a tumor marker. However, elevated CA125 levels are also observed in various non-malignant conditions, including diseases affecting mucosal surfaces, pleural or peritoneal effusions, cirrhosis (with or without ascites), endometriosis, uterine fibroids, adenomyosis, pelvic inflammatory disease, and pregnancy. This review aims to explore the role of CA125 in non-malignant serous effusions, highlighting its diagnostic and prognostic potential beyond the realm of oncology. A comprehensive literature search was conducted across multiple databases and clinical trial registries. Eligible studies included full-text original research articles, reviews, and case reports published in English over the past 10 years. Inclusion criteria were limited to studies involving human subjects and focused on the role of CA125 in non-malignant serous effusions. CA125 is produced by coelomic epithelial cells lining the ovary, pleura, pericardium, and peritoneum. Its serum concentration is not significantly influenced by age, body weight, or renal function, even in the advanced stages of the disease. In peritoneal conditions, CA125 is synthesized by mesothelial cells and serves as a potential marker of peritoneal involvement. The prevailing pathophysiological mechanism suggests that mechanical stretching of mesothelial cells due to ascitic pressure stimulates CA125 release. Similarly, in heart failure, mesothelial cells of the pericardium produce CA125, which correlates with congestion severity, supports risk stratification, and may inform diuretic therapy. While a threshold of 35 U/mL is established for malignancy, no standardized cutoff exists for CA125 in non-malignant conditions. The utility of CA125 measurement in peritoneal, pleural, or pericardial effusions-and cardiovascular diseases such as acute heart failure-for purposes of differential diagnosis, treatment guidance, or prognostication warrants further investigation through prospective clinical trials.