Simple Summary Urothelial carcinoma is a highly aggressive cancer. In addition to the further development and innovation of therapeutic strategies, research into risk and prognostic factors plays a major role. Comparatively little is known about the physical constitution of patients and its impact on the prognosis of the disease. The aim of this retrospective study was to assess specific parameters of the body composition of patients undergoing nephroureterectomy due to urothelial carcinoma of the upper urinary tract, in addition to the usual criteria with regard to commonly used ones. Computed tomography-based measurements were used to determine muscle mass and fat distribution. The cohort included 142 patients. We were able to demonstrate that loss of skeletal muscle mass is a significant comorbidity-independent risk factor. Visceral fat, on the other hand, seems to be protective. In conclusion, specific parameters of body composition can contribute to patient-specific risk stratification. Radical nephroureterectomy (NUE) is the gold standard treatment for high-risk urothelial cancer of the upper urinary tract (UTUC). Besides sarcopenia and frailty, fat distribution is moving increasingly into focus. Components of body composition were assessed in patients undergoing NUE due to UTUC. The study cohort included 142 patients. By using CT-based measurements, the skeletal muscle index (SMI), subcutaneous adipose tissue index (SATI), and visceral adipose tissue index (VATI) were measured at the height of the third lumbar vertebra. Overall survival (OS) and cancer-specific survival (CSS) were estimated using univariable und multivariable Cox regression models. The prevalence of sarcopenia in the study population (n = 142) was 37%. OS and CSS were significantly reduced in sarcopenic patients. In the multivariable cox regression analysis, including age, ACE-27, T-stage, R-stage, LVI and necrosis, sarcopenia remained a significant risk factor of OS (HR, 1.77; 95% CI 1.02-3.07; p = 0.042) and CSS (HR, 2.17; 95% CI 1.18-3.99; p = 0.012). High visceral adipose tissue seems to be protective, although not statistically significant. Sarcopenia is a comorbidity-independent risk factor in patients who underwent NUE due to UTUC. Visceral fat represents a potentially protective factor. These results suggest that specific factors of body composition can be used for better risk stratification.