Pembrolizumab has emerged as the new standard of care in patients with platinum-refractory metastatic urothelial carcinoma (mUC), whereas the optimal risk stratification to predict survival outcomes is still controversial. We examined a risk model for overall survival (OS) in mUC treated with pembrolizumab using our multi-institutional dataset (212 patients). The median age was 72 years old. Median OS from the initiation of pembrolizumab treatment was 11.7 months. The objective response rate (ORR) was 26.4%. On multivariate analysis, multiple metastatic sites and an NLR > 3.50 at the initiation of pembrolizumab treatment were identified as independent predictors for OS. We next developed a risk model using those two predictors. Patients without any factors were assigned to the favorable-risk group (26.5%). Patients with either factor and both factors were assigned to the intermediate-risk group (44.3%), and poor-risk group (29.2%), respectively. Kaplan–Meier curves showed clear discrimination of OS among the risk groups (p < 0.001). The ORR in each group was 35.7% in the favorable-risk group, 27.7% in the intermediate-risk group, and 17.7% in the poor-risk group. Given that the model can be concisely determined at the initiation of pembrolizumab treatment, physicians may be encouraged to consider the risk group for daily practice.