Non-specific low back pain and peripartum pelvic pain have aetiologies that may feature the sacroiliac region. This region possesses many potential pain-generating structures sharing common sensory innervation which makes clinical differentiation of pathoanatomy difficult. This anatomical study explores the relationship between the long posterior sacroiliac ligament (LPSL) and the lateral branches of the dorsal sacral nerve plexus. Twenty-five sides of the pelvis from 16 cadavers were studied, three for histological analysis and 22 for gross anatomical dissection. We found that the LPSL is penetrated by the lateral branches of the dorsal sacral rami of predominantly S2 (96%, 21/22) and S3 (100%, 22/22), variably of S4 (59%, 13/22) and rarely of S1 (4%, 1/22). Some of the penetrating lateral branches give off nerve fibres that disappear within the ligament. These findings provide an anatomical basis for the notion that the LPSL is a potential pain generator in the posterior sacroiliac region.