Background: Postoperative spinal implant infection (PSII) is a clinically significant complication following spinal instrumentation, with diagnostic challenges related to biofilm formation and the lack of standardized criteria. This study aimed to evaluate PSII according to the European Bone and Joint Infection Society (EBJIS) criteria and to identify associated clinical, laboratory, and perioperative risk factors. Methods: In this retrospective cohort study, 224 adult patients who underwent posterior spinal instrumentation surgery between January 2025 and January 2026 were included. Demographic characteristics, comorbidities, laboratory parameters, perioperative variables, and microbiological findings were obtained from electronic medical records. PSII was defined according to the EBJIS “confirmed infection” criteria. Variables with p < 0.10 in univariate analysis were considered for multivariate logistic regression. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated. Results: PSII developed in 18 patients (8.0%). No significant association was found between PSII and demographic variables or comorbidities. Elevated C-reactive protein (CRP) levels and lower serum albumin levels were associated with PSII (p < 0.05). In univariate analysis, ASA score ≥3, emergency surgery, perioperative steroid use, low molecular weight heparin use, prone positioning, and intensive care unit admission were significantly associated with PSII. In multivariate analysis, only emergency surgery remained independently associated with PSII (OR: 6.48; 95% CI: 1.28–32.70; p = 0.024). Receiver operating characteristic (ROC) analysis demonstrated limited discriminative ability of the number of fused vertebrae (AUC = 0.613). Conclusions: PSII remains an important complication following spinal instrumentation surgery. Emergency surgery was independently associated with PSII, while CRP elevation and hypoalbuminemia were identified as associated findings rather than independent predictors. The use of standardized diagnostic approaches such as EBJIS may improve diagnostic consistency and facilitate more reliable risk stratification.