Nitric oxide (NO) stimulates mitochondrial biogenesis in skeletal muscle. However, NO metabolism is disrupted in individuals with type 2 diabetes mellitus (T2DM) potentially contributing to their decreased cardiorespiratory fitness (i.e., VO₂max) and skeletal muscle oxidative capacity. We used a randomized, double-blind, placebo-controlled, 8-week trial with beetroot juice containing nitrate (NO₃⁻) and nitrite (NO₂⁻) (250 mg and 20 mg/day) to test potential benefits on VO₂max and skeletal muscle oxidative capacity in T2DM. T2DM (N = 36, Age = 59 ± 9 years; BMI = 31.9 ± 5.0 kg/m²) and age- and BMI-matched non-diabetic controls (N = 15, Age = 60 ± 9 years; BMI = 29.5 ± 4.6 kg/m²) were studied. Mitochondrial respiratory capacity was assessed in muscle biopsies from a subgroup of T2DM and controls (N = 19 and N = 10, respectively). At baseline, T2DM had higher plasma NO₃⁻ (100%; p < 0.001) and lower plasma NO₂⁻ levels (−46.8%; p < 0.0001) than controls. VO₂max was lower in T2DM (−26.4%; p < 0.001), as was maximal carbohydrate- and fatty acid-supported oxygen consumption in permeabilized muscle fibers (−26.1% and −25.5%, respectively; p < 0.05). NO₃⁻/NO₂⁻ supplementation increased VO₂max (5.3%; p < 0.01). Further, circulating NO₂⁻, but not NO₃⁻, positively correlated with VO₂max after supplementation (R²= 0.40; p < 0.05). Within the NO₃⁻/NO₂⁻ group, 42% of subjects presented improvements in both carbohydrate- and fatty acid-supported oxygen consumption in skeletal muscle (vs. 0% in placebo; p < 0.05). VO₂max improvements in these individuals tended to be larger than in the rest of the NO₃⁻/NO₂⁻ group (1.21 ± 0.51 mL/(kg*min) vs. 0.31 ± 0.10 mL/(kg*min); p = 0.09). NO₃⁻/NO₂⁻ supplementation increases VO₂max in T2DM individuals and improvements in skeletal muscle oxidative capacity appear to occur in those with more pronounced increases in VO₂max.