Background <p>Primary dysmenorrhea is a type of menstrual pain characterized by uterine-origin cramps in the suprapubic region without any underlying pelvic pathology. It is a chronic pain syndrome that negatively affects the quality of life of millions of women worldwide. Pentraxin-3 (PTX3) is a long-chain pentraxin, an acute-phase plasma protein that plays a role in both acute and chronic inflammation and innate immunity. In this study, the diagnostic biomarker value of serum PTX3 levels and the relationship between serum PTX3 and pain were investigated in patients diagnosed with primary dysmenorrhea.</p> Materials and methods <p>This research included 50 patients who presented to the obstetrics and gynecology outpatient clinic and emergency department with complaints of primary dysmenorrhea between July 1, 2025, and September 1, 2025. Moreover, a control group of 50 healthy women, matched with the patient group and without primary dysmenorrhea, was included. PTX3 levels were measured via enzyme-linked immunosorbent assay (ELISA).</p> Results <p>Median PTX3 levels in patients were significantly higher than controls (<i>p</i> &lt; 0.001). PTX3 levels also increased significantly as pain intensity increased (<i>p</i> &lt; 0.001). A significant, strong, and positive correlation was observed between visual analog scale (VAS) scores and PTX3 levels (<i>r</i> = 0.838, <i>p</i> &lt; 0.001). When the cut-off value for PTX3 levels was set at 5.24 ng/ml, sensitivity was 81% and specificity was 95% (AUC = 0.918, <i>p</i> &lt; 0.001).</p> Conclusion <p>In primary dysmenorrhea, PTX3 levels increase with the severity of pain. PTX3 could be an important diagnostic biomarker of primary dysmenorrhea.</p>