Objective To investigate the association of the preoperative systemic immune‐inflammation index (SII) with recurrence‐free survival (RFS) after transurethral resection of the bladder tumor (TURBT) of non‐muscle‐invasive bladder cancer (NMIBC) using propensity score matching (PSM) analysis. Methods The clinicopathological characteristics and follow‐up data of NMIBC patients were collected retrospectively from two tertiary medical centers. A 1:1 PSM analysis was carried out using the nearest‐neighbor method (caliper size: 0.02). Cox regression analysis was used to identify the risk factors associated with RFS. Results A total of 416 NMIBC patients were included in this study. Before and after matching, patients with increased SII had worse RFS ( p < 0.0001 and p = 0.027, respectively). Multivariate Cox analysis identified SII as an independent predictor of RFS before (HR [95% CI]: 1.789 [1.232, 2.599], p = 0.002) and after matching (HR [95% CI]: 1.646 [1.077, 2.515], p = 0.021). In the matched subgroup analysis, an elevated SII had a significant association with postoperative worse RFS in the T1 stage ( p = 0.025), primary status ( p = 0.049), high‐grade ( p = 0.0015), and multiple lesions ( p = 0.043) subgroups. Conclusion SII could accurately stratify the prognosis of NMIBC patients before and after PSM analysis. An elevated SII was significantly associated with worse RFS in NMIBC patients.