Purpose <p>Postoperative ileus (POI) is a frequent and morbid complication after transforaminal lumbar interbody fusion (TLIF). Electrolyte disturbances are implicated in ileus after abdominal surgery, but the role of phosphate in spine surgery is unclear. This study examined whether early postoperative hypophosphatemia is independently associated with POI after TLIF and explored its discriminatory performance and consistency across clinical subgroups.</p> Methods <p>In this multicenter retrospective cohort, adults undergoing primary TLIF for degenerative lumbar disease at two tertiary centres (May 2017–March 2025) were identified. Serum phosphate was measured preoperatively and on postoperative day 1 (POD1). Hypophosphatemia was defined as phosphate &lt; 0.80 mmol/L and further classified as mild (0.60–0.79 mmol/L) or moderate (&lt; 0.60 mmol/L). The primary outcome was POI, defined by persistent intolerance of oral intake, abdominal distension and delayed passage of flatus or stool in the absence of mechanical obstruction. Univariable and multivariable logistic regression were used to estimate odds ratios (ORs) for POI. Discrimination of POD1 phosphate was assessed using the area under the receiver operating characteristic (ROC) curve. Prespecified subgroup analyses were performed for age, diabetes and fusion extent.</p> Results <p>Among 947 included patients, 86 (9.1%) developed POI. Baseline demographics, comorbidities and operative characteristics were similar between groups. Mean POD1 phosphate was lower in patients with POI than in those without POI (0.63 ± 0.14 vs. 0.83 ± 0.17 mmol/L, <i>p</i> &lt; 0.001), and hypophosphatemia was more frequent (88.4% vs. 44.6%, <i>p</i> &lt; 0.001). Compared with normal phosphate, the incidence of POI increased from 2.1% to 11.1% in mild and 33.6% in moderate hypophosphatemia (p for trend &lt; 0.001). POD1 hypophosphatemia was strongly associated with POI (univariable OR 9.44, 95% CI 4.82–18.50), and this association persisted after adjustment for age ≥ 70 years, sex, diabetes, chronic constipation, prior abdominal surgery and fusion ≥ 3 levels, opioid dose, fluid balance on POD1 and mFI-5. POD1 phosphate showed good discrimination for POI (AUC 0.82, 95% CI 0.76–0.87). A cutoff of 0.74 mmol/L provided 79.1% sensitivity, 70.0% specificity and a negative predictive value of 97.1%. The association between hypophosphatemia and POI was generally consistent across strata defined by age, diabetes status and fusion extent, with no convincing evidence of effect modification.</p> Conclusions <p>Early postoperative hypophosphatemia on POD1 is a strong, independent predictor of POI after TLIF, with a clear dose–response relationship and good discriminatory performance. Routine monitoring of POD1 phosphate and consideration of targeted correction may help identify and manage patients at increased risk of ileus and prolonged hospitalization after lumbar fusion. Prospective studies are warranted to determine whether phosphate-guided perioperative strategies can improve gastrointestinal recovery.</p>