Background: Guidelines on heart failure (HF) define iron deficiency (ID) as a serum ferritin <100 ng/mL or, when 100-299 ng/mL, a transferrin saturation (TSAT) <20%. Inflammation (common in HF) may hinder interpretation of serum ferritin. Objectives: This study sought to investigate how different definitions of ID affect its prevalence and relationship to prognosis in ambulatory patients with chronic HF. Methods: Prevalence, relationship with patients’ characteristics, and outcomes of various ID definitions were evaluated among patients with HF referred to a regional clinic (Hull LifeLab) from 2001 to 2019. Results: Of 4,422 patients with HF (median age 75 years [range: 68-82 years], 60% men, 32% with reduced left ventricular ejection fraction), 46% had TSAT <20%, 48% had serum iron ≤13 μmol/L, 57% had serum ferritin <100 ng/mL, and 68% fulfilled current guideline criteria for ID, of whom 35% had a TSAT >20%. Irrespective of definition, ID was more common in women and those with more severe symptoms, anemia, or preserved ejection fraction. TSAT <20% and serum iron ≤13 μmol/L, but not guideline criteria, were associated with higher 5-year mortality (HR: 1.27; 95% CI: 1.14-1.43; P < 0.001; and HR: 1.37; 95% CI: 1.22-1.54; P < 0.001, respectively). Serum ferritin <100 ng/mL tended to be associated with lower mortality (HR: 0.91; 95% CI: 0.81-1.01; P = 0.09). Conclusions: Different definitions of ID provide discordant results for prevalence and prognosis. Definitions lacking specificity may attenuate the benefits of intravenous iron observed in trials while definitions lacking sensitivity may exclude patients who should receive intravenous iron. Prespecified subgroup analyses of ongoing randomized trials should address this issue.