In locally advanced and metastatic malignancies, antibiotic (ATB) therapy has a negative effect on immunotherapy efficacy. Therefore, we aimed to evaluate whether ATB therapy and use of specific ATB classes with concomitant neoadjuvant pembrolizumab affected pathologic complete response (ypT0N0) and relapse-free survival (RFS) for patients with clinical T2–4N0M0 bladder cancer enrolled in the PURE-01 study. Of the 149 patients evaluated, 48 (32%) received any concomitant ATB therapy. The ATB class most commonly administered was fluoroquinolones (16 patients; 33%). In the ATB cohort, seven patients (15%) achieved ypT0N0 status, compared to 50 (50%; p < 0.001) in the untreated group. Moreover, ATB use was negatively associated with ypT0N0 status (odds ratio 0.18, 95% confidence interval [CI] 0.05–0.48; p = 0.001). The 24-mo RFS rate was 63% (95% CI 48-83%) in the ATB group versus 90% (95% CI 83–97) in the untreated group. We found that ATB use was associated with a higher recurrence rate (hazard ratio [HR] 2.64, 95% CI 1.08–6.50; p = 0.03). Exploratory analyses showed that fluoroquinolone use was associated with a higher recurrence rate (HR 3.28, 95% CI 1.12–9.60; p = 0.03). Our study revealed an association between ATB use and neoadjuvant immunotherapy efficacy in an intention-to-cure population, highlighting the need for future studies to better investigate this relationship.