Background: Sodium-glucose cotransporter 2 (SGLT2) inhibitors reduce the urinary albumin-to-creatinine ratio (UACR) in patients with elevated levels of albuminuria in the presence or absence of heart failure (HF) or type 2 diabetes mellitus (T2D). However, these effects have not yet been reported in the presence of both HF and T2D. This lack of evidence prompted us to conduct a clinical trial on the effects of dapagliflozin on UACR in patients with HF and T2D. Methods: DAPPER is a multicentre, randomised, open-labeled, parallel-group, standard treatment-controlled trial that enrolled patients at 18 medical facilities in Japan. Eligible participants with both HF and T2D and aged between 20 and 85 years were randomly assigned to a dapagliflozin or control (anti-diabetic drugs other than SGLT 2 inhibitors) group with a 1:1 allocation. The primary outcome was changes in UACR from baseline after a two-year observation, and secondary endpoints were cardiovascular (CV) events and parameters related to HF. This trial was registered with the UMIN-CTR registry, UMIN000025102 and the Japan Registry of Clinical Trials, jRCTs051180135. Findings: Between 12 May 2017 and 31 March 2020, 294 patients were randomly assigned to the dapagliflozin group (n = 146) or control group (n = 148). The mean age of patients was 72.1 years and 29% were female. The mean glycated hemoglobin value was 6.9%, mean NT-proBNP was 429.1 pg/mL, mean estimated GFR was 65.7 mL/min/1.73 m Interpretation: Although dapagliflozin at a dose of 5 mg daily did not reduce urinary albumin excretion in patients with HF and T2D from that in the controls, our findings suggest that dapagliflozin decreased CV events and suppressed left ventricular remodeling. Funding: AstraZeneca KK, Ono Pharmaceutical Co., Ltd.