The day of the week of admission may influence the length of stay and in-hospital death. However, the association between the admission day of the week and in-hospital outcomes has been inconsistent in heart failure (HF) patients among studies reported from Western countries. We thus analyzed this association in HF patients encountered in routine clinical practice in Japan. We studied the characteristics and in-hospital treatment in 1620 patients hospitalized with worsening HF by using the database of the Japanese Cardiac Registry of Heart Failure in Cardiology (JCARE-CARD). Patients were divided into two groups according to weekday (n = 1355; 83.6%) or weekend admission (n = 265; 16.4%). The mean age was 70.7 years and 59.4% were male. Etiology was ischemic in 34.0%, and mean left ventricular ejection fraction was 42.5%. Patients admitted on the weekend were significantly older and had more comorbidities, and more severe symptoms and signs of HF on admission. Length of stay was comparable between weekend and weekday admission (35.2 ± 47.0 days vs 33.6 ± 32.0 days, P = 0.591). Crude in-hospital mortality did not differ between patients admitted on the weekend and weekdays (7.5% vs 5.2%, P = 0.136). Even after adjustment for covariates in multivariable modeling with patients admitted on weekday as the reference, in-hospital death was comparable between patients admitted on the weekend and weekdays (adjusted odds ratio 1.125, 95% confidence interval 0.631-2.004, P = 0.691). Among patients hospitalized for worsening HF, admission day of the week did not affect in-hospital death and length of stay.