The ALIMENT-HF trial aims to determine whether high-calorie and high-protein oral nutritional supplements (ONS) are safe and beneficial for older adult outpatients with heart failure (HF). This multicentre, single-arm, interventional pilot trial is designed to evaluate the tolerance, efficacy, and safety of ONS in older adult outpatients with chronic HF, malnutrition, and anorexia. In total, 80 outpatients with HF regardless of their left ventricular ejection fraction will be treated with ONS, including high-energy (900kcal/day) and high protein (36g/day) supplementation, at eight sites in Japan. Inclusion criteria are as follows: age, ≥65years; outpatients receiving maximally tolerated guideline-directed medical therapy for HF and without change in their diuretic dosage during the last 3months; outpatients at risk of malnutrition, defined as a Malnutrition Universal Screening Tool score ≥1 point, and anorexia, defined using a Simplified Nutritional Appetite Questionnaire for the Japanese Elderly (SNAQ-JE) score of ≤14 points. Nutritional intervention will continue for up to 120days, with an observational period lasting for a further 60days. The primary outcome is a change in body weight between baseline and day 120. The ALIMENT-HF trial will evaluate the tolerance, efficacy, and safety of high-calorie and high-protein-rich ONS in older outpatients with HF co-morbid with malnutrition and anorexia and will provide insightful information for future randomized controlled trials.