Diastolic dysfunction (DD) in heart failure is associated with increased myocardial cytosolic calcium and calcium-efflux through the sodium–calcium exchanger depends on the sodium gradient. Beta-3-adrenoceptor (β3-AR) agonists lower cytosolic sodium and have reversed organ congestion. Accordingly, β3-AR agonists might improve diastolic function, which we aimed to assess. In a first-in-man, randomized, double-blinded trial, we assigned 70 patients with HF with reduced ejection fraction, New York Heart Association II-III, and left ventricular ejection fraction <40% to receive the β3-AR agonist mirabegron (300 mg/day) or placebo for 6 months, in addition to recommended heart failure therapy. We performed echocardiography and cardiac computed tomography and measured N-terminal probrain natriuretic peptide at baseline and follow-up. DD was graded per multiple renowned algorithms. Baseline and follow-up data were available in 57 patients (59 ± 11 years, 88% male, 49% ischemic heart disease). No clinically significant changes in diastolic measurements were found within or between the groups by echocardiography (E/e′ placebo: 13 ± 7 to 13 ± 5, P = 0.21 vs. mirabegron: 12 ± 6 to 13 ± 8, P = 0.74, between-group follow-up difference 0.2 [95% CI, −3 to 4], P = 0.89) or cardiac computed tomography (left atrial volume index: between-group follow-up difference 9 mL/m2 [95% CI, −3 to 19], P = 0.15). DD gradings did not change within or between the groups following 2 algorithms (P = 0.72, P = 0.75). N-terminal probrain natriuretic peptide remained unchanged in both the groups (P = 0.74, P = 0.64). In patients with HF with reduced ejection fraction, no changes were identified in diastolic measurements, gradings or biomarker after β3-AR stimulation compared with placebo. The findings add to the previous literature questioning the role of impaired Na+-Ca2+–mediated calcium export as a major culprit in DD.