The clinical importance of nutritional management in activities of daily living (ADL) among older inpatients with heart failure (HF) is greatly increasing. We determined the optimal nutritional assessment tool that can predict ADL decline among older inpatients with HF. We prospectively investigated 91 inpatients aged ≥ 65 years with HF in an acute hospital. We measured their nutritional status at admission using nutrition indices: the controlling nutritional status (CONUT), the geriatric nutritional risk index, the prognostic nutritional index, and the mini nutritional assessment. Logistic regression analysis was used to estimate the odds ratios (ORs) and 95% confidence intervals (CIs) of the relationships between the malnutrition status assessed by each nutritional index category and the ADL decline measured by the Barthel index (BI) in the univariate and multivariate analyses. Among the participants, 28.6% (n = 26; median age 81.5 years; 69.2% men) of the participants were included in the Reduced BI group and 71.4% (n = 65; median age 79.0 years; 67.7% men) in the Maintained BI group. The Reduced BI group showed a significantly higher CONUT value than the Maintained BI group, but there were no significant differences in other nutritional indices. In the multivariate logistic regression analysis, a higher CONUT score was associated with a significantly elevated risk of Reduced BI (adjusted OR 0.24; 95%CI 0.08–0.75; p = 0.014). We found that CONUT is an appropriate nutritional assessment tool for predicting ADL decline among older inpatients with HF in the early phase of hospitalization.