Abstract Background Most studies on the association between the frailty syndrome and adverse health outcomes in patients with heart failure (HF) have used non-standard definitions of frailty. This study examined the association of frailty, diagnosed by well-accepted criteria, with mortality, readmission and functional decline in very old ambulatory patients with HF. Methods Prospective study with 497 patients in six Spanish hospitals and followed up during one year. Mean (SD) age was 85.2 (7.3) years, and 79.3% had LVEF > 45%. Frailty was diagnosed as having ≥ 3 of the 5 Fried criteria. Readmission was defined as a new episode of hospitalisation lasting > 24 h, and functional decline as an incident limitation in any activity of daily living at the 1-year visit. Statistical analyses were performed with Cox and logistic regression, as appropriate, and adjusted for the main prognostic factors at baseline. Results At baseline, 57.5% of patients were frail. The adjusted hazard ratio (95% confidence interval) for mortality among frail versus non-frail patients was 1.93 (1.20–3.27). Mortality was higher among patients with low physical activity 1.64 (1.10–2.45) or exhaustion 1.83 (1.21–2.77). Frailty was linked to increased risk of readmission 1.66 (1.17–2.36) and functional decline odds ratio 1.67 (1.01–2.79). Slow gait speed was related to functional decline odds ratio 3.59 (1.75–7.34). A higher number of frailty criteria was associated with a higher risk of the three study outcomes (P trend < 0.01 in each outcome). Conclusions Frailty was associated with increased risk of 1-year mortality, hospital readmission and functional decline among older ambulatory patients with HF.