Agenesis of the piriformis muscle is an extremely rare occurrence. Knowledge about this anatomic variant is important because of its close proximity with the sciatic nerve and sacral plexus. The piriformis muscle also serves as an important anatomic landmark for image-guided intervention and hip surgery. We report a case of piriformis muscle agenesis in a 28-year-old woman, incidentally detected on magnetic resonance imaging of the lumbosacral spine and pelvis, performed for low back pain.