: Upper tract urothelial carcinoma (UTUC) is often complicated by intravesical recurrence and cancer progression following radical nephroureterectomy (RNU). Identifying reliable prognostic biomarkers remains crucial for optimizing postoperative surveillance. The goal of this study was to assess the prognostic value of the De Ritis ratio (AST/ALT) in predicting bladder recurrence and oncologic outcomes in patients with clinically localized UTUC undergoing RNU. : This retrospective study analyzed 87 patients treated with RNU between 2018 and 2025. Preoperative De Ritis ratios were calculated, and an optimal cut-off value of 1.682 was determined using ROC analysis. Recurrence-free survival (RFS), cancer-specific survival (CSS), and overall survival (OS) were analyzed using the Kaplan-Meier and Cox regression methods. Logistic regression was used to identify independent predictors of bladder recurrence. : A high De Ritis ratio was significantly associated with increased bladder recurrence and worse RFS and CSS, but not OS. Multivariate analysis confirmed that an elevated De Ritis ratio, current smoking, positive surgical margins, and synchronous bladder cancer were the independent predictors of bladder recurrence. The De Ritis ratio demonstrated strong discriminatory performance (AUC: 0.807), with good sensitivity and specificity for predicting recurrence. : The De Ritis ratio is a simple, cost-effective preoperative biomarker that may aid in identifying UTUC patients at higher risk for intravesical recurrence and cancer-specific mortality. Incorporating this ratio into clinical decision-making could enhance risk stratification and guide tailored follow-up strategies.