The effects of empagliflozin on cardiorespiratory fitness in patients with type 2 diabetes mellitus (T2DM) and heart failure with reduced ejection fraction (HFrEF) are unknown. In this pilot study we determined the effects of empagliflozin 10 mg/d for 4 weeks on peak oxygen consumption (VO₂) in 15 patients with T2DM and HFrEF. As an exploratory analysis, we assessed whether there was an interaction of the effects of empagliflozin on peak VO₂ of loop diuretics. Empagliflozin reduced body weight (−1.7 kg; P = .031), but did not change peak VO₂ (from 14.5 mL kg⁻¹ min⁻¹ [12.6‐17.8] to 15.8 [12.5‐17.4] mL kg⁻¹ min⁻¹; P = .95). However, patients using loop diuretics (N = 9) demonstrated an improvement, whereas those without loop diuretics (N = 6) experienced a decrease in peak VO₂ (+0.9 [0.1‐1.4] vs −0.9 [−2.1 to −0.3] mL kg⁻¹ min⁻¹; P = .001), and peak VO₂ changes correlated with the baseline daily dose of diuretics (R = +0.83; P < .001). Empagliflozin did not improve peak VO₂ in patients with T2DM and HFrEF. However, as a result of exploratory analysis, patients concomitantly treated with loop diuretics experienced a significant improvement in peak VO₂.