<b>Background:</b> Heart failure with a preserved ejection fraction (HFpEF) accounts for approximately 50% of patients with heart failure. Endothelial dysfunction has been documented in HFpEF, and impaired nitric oxide (NO) production may be a contributing factor in patients decompensating from chronic to acute HFpEF. Plasma nitrite (NO₂^-), but not plasma nitrate (NO₃^-), is highly reflective of local nitric oxide production. However, plasma NO₂^- levels in relation to chronic and acute HFpEF patients have not been studied. <b>Methods:</b> Plasma NO₂^- and NO₃^- concentrations were quantified with gas-phase chemiluminescence. Plasma asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO production, and arterial stiffness were also quantified. Samples were collected from 19 participants with chronic HFpEF and 16 participants with acute HFpEF. <b>Results:</b> Plasma NO₂^- concentrations were lower in participants with acute HFpEF when compared to the chronic HFpEF group (<i>p</i> = 0.022). NO₃^-, ADMA and indices of arterial stiffness did not display any significant between-group differences. <b>Conclusions:</b> We present novel NO₂^- data, which has not been previously quantified in patients with acute HFpEF. Our results indicate that plasma concentrations of NO₂^- may be decreased in patients with acute HFpEF compared to patients with chronic HFpEF, suggesting a dysregulated NO pathway. Further studies are required to confirm these findings in this patient population.