BACKGROUND: The morbidity and mortality rates of patients with heart failure (HF) and functional mitral regurgitation (MR) remain substantial despite guideline-directed medical therapy for HF. We evaluated the efficacy of ertugliflozin for reduction of functional MR associated with HF with mild to moderately reduced ejection fraction. METHODS: The EFFORT trial (Ertugliflozin for Functional Mitral Regurgitation) was a multicenter, double-blind, randomized trial to examine the hypothesis that the sodium-glucose cotransporter 2 inhibitor ertugliflozin is effective for improving MR in patients with HF with New York Heart Association functional class II or III, 35%≤ejection fraction<50%, and effective regurgitant orifice area of chronic functional MR >0.1 cm ² on baseline echocardiography. We randomly assigned 128 patients to receive either ertugliflozin or placebo in addition to guideline-directed medical therapy for HF. The primary end point was change in effective regurgitant orifice area of functional MR from baseline to the 12-month follow-up. Secondary end points included changes in regurgitant volume, left ventricular (LV) volume indices, left atrial volume index, LV global longitudinal strain, and NT-proBNP (N-terminal pro-B-type natriuretic peptide). RESULTS: The treatment groups were generally well-balanced with regard to baseline characteristics: mean age, 66±11 years; 61% men; 13% diabetes; 51% atrial fibrillation; 43% use of angiotensin receptor-neprilysin inhibitor; ejection fraction, 42±8%; and effective regurgitant orifice area, 0.20±0.12 cm ² . The decrease in effective regurgitant orifice area was significantly greater in the ertugliflozin group than in the placebo group (−0.05±0.06 versus 0.03±0.12 cm ² ; P <0.001). Compared with placebo, ertugliflozin significantly reduced regurgitant volume by 11.2 mL (95% CI, −16.1 to −6.3; P =0.009), left atrial volume index by 6.0 mL/m ² (95% CI, −12.16 to 0.15; P =0.005), and LV global longitudinal strain by 1.44% (95% CI, −2.42% to −0.46%; P =0.004). There were no significant between-group differences regarding changes in LV volume indices, ejection fraction, or NT-proBNP levels. Serious adverse events occurred in one patient (1.6%) in the ertugliflozin group and 6 (9.2%) in the placebo group ( P =0.12). CONCLUSIONS: Among patients with functional MR associated with HF, ertugliflozin significantly improved LV global longitudinal strain and left atrial remodeling, and reduced functional MR. Sodium-glucose cotransporter 2 inhibitors may be considered for patients with functional MR. REGISTRATION: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT04231331.