Background: Low 24-h urinary excretion of creatinine in patients with heart failure (HF) is believed to reflect muscle wasting and is associated with a poor prognosis. Recently, spot urinary creatinine concentration (SUCR) has been suggested as a useful prognostic factor in selected HF cohorts. This more practical and cheaper approach has never been tested in an unselected HF population. Moreover, neither the relation between SUCR and body composition markers nor the association of SUCR with the markers of volume overload, which are known to worsen clinical outcome, has been studied so far. The aim of the study was to check the prognostic value of SUCR in HF patients after adjusting for body composition and indirect markers of volume overload. Methods: In 911 HF patients, morning SUCR was determined and body composition scanning using dual X-ray absorptiometry (DEXA) was performed. Univariable and multivariable predictors of log SUCR were analyzed. All participants were divided into quartiles of SUCR. Results: In univariable analysis, SUCR weakly correlated with fat-free mass (R = 0.09,p= 0.01). Stronger correlations were shown between SUCR and loop diuretic dose (R = 0.16,p< 0.0001), NTproBNP (R = −0.15,p< 0.0001) and serum sodium (R = 0.16,p< 0.0001). During 3 years of follow-up, 353 (38.7%) patients died. Patients with lower SUCR were more frequently female, and their functional status was worse. The lowest mortality was observed in the top quartile of SUCR. In the unadjusted Cox regression analysis, the relative risk of death in all three lower quartiles of SUCR was higher by roughly 80% compared to the top SUCR quartile. Apart from lower SUCR, the significant predictors of death were age and malnutrition but not body composition. After adjustment for loop diuretic dose and percent of recommended dose of mineralocorticoid receptor antagonists, the difference in mortality vanished completely. Conclusions: Lower SUCR levels in HF patients are associated with a worse outcome, but this effect is not correlated with fat-free mass. Fluid overload-driven effects may link lower SUCR with higher mortality in HF.