Background: We conducted a randomized, controlled trial to determine whether supplementation with oral branched-chain amino acids (BCAAs) improves serum albumin and clinical outcomes in heart failure (HF) patients with hypoalbuminemia. Methods and results: We randomly assigned 18 in-hospital HF patients with serum albumin < 3.5 g/dL to receive oral BCAA granules (LIVACT®) for 28 days during their hospital stay or until discharge (BCAA group; N = 9) or to receive no supplementation (controls; N = 9), in addition to recommended HF therapy. The primary endpoints were changes from baseline in serum albumin and cardiothoracic ratio (CTR). Sixteen patients completed the study. The mean (± standard deviation) period of BCAA supplementation was 18.4 ± 8.4 days. Serum albumin significantly increased in the BCAA group [mean difference vs baseline, 0.44 g/dL; 95% confidence interval (CI) 0.13–0.76; P = 0.014] and did not change in controls (0.18 g/dL; 95% CI − 0.05 to 0.40; P = 0.108). CTR significantly decreased in the BCAA group (− 2.3%; 95% CI − 3.8 to − 0.8; P = 0.014) and did not change in controls (− 1.0%; 95% CI − 2.3 to 0.3; P = 0.111). Conclusion: In-hospital HF patients with hypoalbuminemia supplemented with BCAAs showed increased serum albumin and decreased CTR. Clinical trial registration number UMIN000004488 [http://www.umin.ac.jp/ctr/index.htm].