The long posterior sacro-iliac ligament (LPSL) is directly posterior to the sacro-iliac joint and a potential source of lower back and pelvic pain. Its sonographic anatomy has not been described in detail. The aim of this study was to define and measure the ligament in healthy young women using ultrasound (US). The LPSL was scanned in 30 healthy women (median age, 22 years; range, 20-34) using a high-resolution linear transducer (7.5-10 MHz). The ligament was consistently visualized as a hyperechoic laminated linear structure between the posterior superior iliac spine and the lateral aspect of the third transverse sacral tubercle. Its length, thickness, and the angle between it and the posterior superior iliac spine were measured by an experienced sonographer bilaterally in both semiflexed standing and lateral decubitus positions. Four female cadaver pelves (age range, 57-93 years) were also scanned and dissected to validate US observations. In the semiflexed standing position, mean LPSL length was 37.9 ± 2.4 mm, mean thickness 1.57 ± 0.38mm, and median angle 18.5°. There was no statistically significant difference with equivalent values in the lateral decubitus position. Intrarater repeatability was fair to substantial in both positions (intraclass correlation coefficient, 0.39-0.66), improving to moderate to substantial (intraclass correlation coefficient, 0.57-0.80) using the mean of two measurements. There was good overall agreement between LPSL length and thickness in cadavers measured by US and dissection. These findings document the sonographic appearance, length, and thickness of the LPSL and provide useful normative data for understanding potential LPSL pathology, particularly in relation to pregnancy-related pelvic girdle pain.