Background Data: The rationale behind lumbar fusion surgery is to eliminate pathologic segmental motion and its accompanying symptoms, especially low back pain.Posterolateral fusion (PLF) using pedicle screw fixation (PSF) has been one of the most popular procedures among the posterior lumbar reconstruction techniques.Lumbar interbody fusion is a recognized surgical technique in treating chronic low back pain in segmental instability. Purpose: The purpose of this study was to compare the clinical and radiological outcomes of stabilizing the lumbar spine using TLIF versus PLF for lumbar segmental instability. Study Design: A prospective, nonrandomized clinical controlled trial. Patients and Methods: Forty patients with segmental lumbar instability were divided into two groups: TLIF and PLF groups, with twenty patients each. Top loaded pedicle screw construct was used with both groups. The mean age of the patients was 48.35 years in the TLIF group and 45.3 years in the PLF group. Sex distribution was 6 males and 14 females in the TLIF group and 7 males and 13 females in the PLF group. Mechanical low back pain was the chief complaint in all patients. Sciatica was a complaint in 12 patients (60%) of the TLIF group and 13 (65%) patients of the PLF group. Patients were evaluated pre- and postoperatively by visual analogue scale (VAS), Oswestry disability index (ODI), and radiographs. Results: The average operative time was 214.5 minutes in the TLIF group and 192.5 minutes in the PLF group. The mean estimated blood loss was 278 ml in the TLIF group and 259 ml in the PLF group. The average length of hospital stay was 3.85 days in the TLIF group and 3.8 days in the PLF group. Patients progressively improved regarding VAS and ODI in both groups with no statistically significant difference except for VAS for back pain, where the TLIF group gave better results. However, TLIF group gave better results in patients with postlaminectomy instability than the PLF group. Solid fusion occurred in 17 patients (85%) of the TLIF group and 16 patients (80%) of the PLF group with no statistical difference. Conclusion: Both transforaminal lumbar interbody fusion and posterolateral fusion are effective and safe options for treating segmental lumbar instability. However, interbody fusion yield superior results in patients with postlaminectomy instability. (2021ESJ253)