Purpose <p>To identify factors contributing to spontaneous facet fusion (SFF) after lateral lumbar interbody fusion (LLIF) with percutaneous pedicle screw fixation and evaluate its clinical significance.</p> Methods <p>We analyzed 43 patients (150 facet joints) who underwent LLIF with percutaneous pedicle screw fixation without posterior decompression and had at least one year of follow-up. Factors associated with SFF included patient-related factors (age, BMI, diabetes, smoking, osteoporosis), lumbar spinal factors (preoperative diagnosis, number of fused segments), and radiologic factors (facet osteoarthritis, interbody fusion status, cage position, and subsidence). Clinical outcomes were assessed using VAS for lower back pain (LBP) and radiating pain (RP) and the Oswestry Disability Index (ODI). Patients were categorized into three groups based on facet and interbody fusion status.</p> Results <p>SFF was observed in 65.3% (98/150) of facet joints, and in 52.0% of cases, SFF preceded interbody fusion. Preoperative facet osteoarthritis (<i>p</i> = 0.009, OR = 9.951) and solid interbody fusion (<i>p</i> = 0.048, OR = 9.837) were significant predictors of SFF. Clinical analysis showed significantly lower VAS for LBP in fused segments (<i>p</i> = 0.023), though no difference between facet fusion-only and interbody fusion groups.</p> Conclusions <p>SFF frequently occurs after LLIF with percutaneous pedicle screw fixation, often before interbody fusion. It was more common in preoperative facet joint osteoarthritis and was frequently seen with solid interbody fusion. SFF correlates with improved LBP outcomes.</p>