INTRODUCTION: Proton pump inhibitors (PPI) may influence the gut microbiome and thus impact the effectiveness of immune checkpoint inhibitors (ICI). The effect of PPIs on the outcomes of ICI has not been fully explored and investigated in metastatic renal cell carcinoma (mRCC). METHODS: This retrospective analysis used prospectively collected data from the GETUG-AFU 26 NIVOREN (NCT03013335) phase II study which enrolled 729 mRCC patients of whom 720 were treated with nivolumab. The main objective of this analysis was to evaluate the impact of PPI on the efficacy and safety outcomes of mRCC patients. PPI use was defined as PPI administration on the day of ICI initiation. RESULTS: Of the 707 patients with mRCC analyzed in this study, 196 (27.7%) were PPI users. The majority of PPI users were males (80.6%), had an ECOG performance status of 0-1 (78.9%) and a nephrectomy (82.1%). Almost two-thirds of the patients had a favorable and intermediate IMDC risk category and 52% received nivolumab in the third line and beyond. PPI use did not correlate with PFS or OS (HR = 0.89, 95% CI 0.74-1.08 and HR = 1.24; 95% CI, 0.98-1.58, respectively). Grade 3-5 nivolumab-related adverse events were more common among PPI users (25.5% vs. 15.3%). CONCLUSIONS: This real-world study suggests that PPI use in patients with mRCC does not impact the efficacy outcomes but may influence the safety of nivolumab which warrants further investigations.