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Background/Objectives: Commonly characterized by limited metastatic sites, low tumor burden has been associated with favorable patient outcomes in various malignancies. However, its prognostic relevance in avelumab maintenance therapy for advanced urothelial carcinoma (UC) remains incompletely defined. Methods: We retrospectively analyzed 26 patients with advanced UC who received avelumab maintenance therapy following disease control with first-line platinum-based chemotherapy between March 2021 and May 2025. Patients were categorized by their metastatic pattern at chemotherapy initiation: lymph node-only (as a surrogate for low tumor burden), non-visceral (excluding visceral organ involvement, but including bone), or visceral disease. Survival outcomes were assessed using the Kaplan–Meier method. Results: Among the cohort, 46.2% had lymph node-only metastasis and 57.7% had non-visceral disease. The median progression-free survival (PFS) and overall survival (OS) from the start of avelumab were 5.6 months and 21.7 months, respectively. OS from the initiation of platinum-based chemotherapy was 28.7 months. Patients with lymph node-only metastasis demonstrated significantly longer OS from chemotherapy initiation compared with those with other metastatic patterns (41.1 vs. 22.9 months, P = 0.044). However, the PFS and OS from avelumab initiation did not significantly differ. No survival benefit was observed for patients with non-visceral disease compared with those with visceral metastases. Conclusions: Lymph node-only metastasis, representing low tumor burden, was associated with significantly improved long-term survival in advanced UC patients undergoing avelumab maintenance following chemotherapy. These findings support the clinical utility of baseline tumor burden and metastatic pattern in risk stratification and shared decision-making for maintenance therapy in advanced UC.