Background/Objectives: Dysplastic spondylolisthesis is associated with developmental abnormalities of the lumbosacral junction, but the contribution of extremely sagittal L5-S1 facet orientation in an adolescent with an intact pars interarticularis remains incompletely characterized. We aimed to describe this unusual quantitative morphologic phenotype and discuss its possible biomechanical implications. Methods: Clinical records and serial imaging of a 14-year-old girl with symptomatic low-grade L5-S1 dysplastic spondylolisthesis were reviewed. Standing radiographs, flexion-extension radiographs, computed tomography (CT), magnetic resonance imaging (MRI), quantitative spinopelvic parameters, intraoperative findings, treatment, and 12-month follow-up were assessed. Results: The patient had approximately one year of progressively worsening low back pain and occasional transient right-leg numbness despite more than 8 months of lumbosacral bracing and more than 6 months of supervised physical therapy. Standing radiographs showed Meyerding grade I L5-S1 anterolisthesis and mild thoracolumbar scoliosis. Quantitative spinopelvic assessment showed a pelvic incidence (PI) of 76.63°, pelvic tilt of 22.21°, sacral slope of 54.42°, lumbar lordosis of 59.5°, sagittal vertical axis of 2.67 cm, and a Dubousset lumbosacral angle of 109°. CT demonstrated an intact pars interarticularis, bilateral facet subluxation, and L5-S1 facet angles of 6° on the left and 3° on the right relative to the sagittal plane. Flexion-extension radiographs showed measurable L5-S1 angular motion from −6.7° to +3.5°, while MRI showed no major central canal stenosis, focal disc herniation, or definite neural compression. Posterior reduction and L5-S1 fusion were performed after individualized counseling, and facet subluxation was confirmed intraoperatively. At 6 and 12 months, the patient remained free of recurrent pain or numbness and radiographs showed maintained lumbosacral alignment. Conclusions: Quantitatively extreme L5-S1 facet sagittalization accompanied by bilateral facet subluxation, an intact pars, and high-PI low-grade spondylolisthesis represents a distinctive morphologic phenotype. This configuration is biomechanically compatible with reduced resistance to anterior shear, although a single case cannot establish independent causality or justify a general indication for early surgery. Comparative quantitative imaging and longitudinal studies are required.