Abstract Purpose To investigate the prognostic value of preoperative serum albumin to globulin ratio (AGR) in patients with non-muscle-invasive bladder cancer (NMIBC) treated with transurethral resection of bladder tumor (TURB) with or without intravesical therapy (IVT). Materials and methods We retrospectively reviewed 1,096 consecutive patients with NMIBC. Levels of albumin and globulin were obtained before TURB and used to calculate the preoperative AGR level. Multivariable Cox regression analyses were performed to assess the prognostic effect of preoperative AGR on oncologic outcomes. Subgroup analyses were performed in patients based on the European Association of Urology (EAU) risk groups for NMIBC. Results Low AGR levels were observed in 389 (35.5%) patients. The median follow-up was 63.7 months (IQR 25.3–111). On multivariable Cox regression analysis, low AGR was associated with increased risk of progression to muscle-invasive BCa (MIBC) (HR 1.81, 95% CI 1.22–2.68, P = 0.003). The addition of AGR only minimally improved the discrimination ability of a base model that included established clinicopathologic features (C-index = 0.7354 vs. C-index = 0.7162). Low preoperative AGR was not significantly associated with the risk of disease recurrence (P = 0.31). In subgroup analyses based on patients’ EAU risk groups, low preoperative AGR was not associated with recurrence-free survival (RFS) (P = 0.59) or progression-free survival (PFS) (P = 0.22) in any of the risk groups. Additionally, in patients treated with Bacillus Calmette–Guerin (BCG) for intermediate- or high-risk NMIBC, low AGR failed to predict disease recurrence or progression. Conclusion Preoperative serum AGR levels independently predicted the risk of disease progression in patients with NMIBC. However, it was not found to be associated with either RFS or PFS in NMIBC patients based on their EAU risk group. This marker seems to have a limited role in NMIBC at the present time. However, further research is needed to investigate this marker in combination with other systemic inflammatory markers to help improve prediction in this heterogeneous group of patients.