INTRODUCTION. Psoas quartus has been described with proximal attachments to quadratus lumborum and transverse process of L3 and distally to the lesser trochanter with the iliopsoas tendon. The current variation presented with multiple bellies, altered proximal attachments, and an entrapment site involving L3 ventral rami. RESOURCES. Typical dissection of the inguinal and pelvic regions of an embalmed 83-year-old, Caucasian female during the Physical Therapy anatomy course revealed the anomalous psoas quartus muscle on the left side. DESCRIPTION. The key discovery was a double bellied psoas quartus muscle that attached distally to the lesser trochanter with the combined tendon of the iliopsoas. Proximal attachments for the medial belly were from the mammillary processes and transverse processes of the lumbar vertebrae, L3- L4 and for the lateral belly from the pelvic surface of the iliac crest and the quadratus lumborum. The ventral rami of L3 passed through the medial belly via a tendinous tunnel created by the muscle. The muscle bellies joined 14 cm distal to the L3 attachment point and extended together another 13 cm in an infero-lateral direction spanning the iliac fossa before joining with the iliopsoas tendon. The muscle’s width measured 31 mm from medial to lateral at its central point. The variant psoas quartus was positioned antero-medial to the iliacus muscle, and posterior to the psoas major muscle. Distally, the muscle belly gave way to a slender tendon that coursed between the tendons of the iliacus and psoas major muscles, delaying the typical conjoining of the iliacus and psoas major tendons. SIGNIFICANCE. The variation in the present case could have many clinical implications including entrapment of lumbar nerve root L3, altered abdominal imaging, altered hip function, along with possible disc herniation or back pain due to increased tension on transverse processes of L3 and L4.