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Background: The effect of glycated hemoglobin (HbA 1c ) variability on adverse outcomes in patients with heart failure (HF) is unclear. We aim to investigate the predictive value of HbA 1c variability on the risks of all-cause death and HF rehospitalization in patients with HF irrespective of their diabetic status., Methods and Results: Using a previously validated territory-wide clinical data registry, HbA 1c variability was assessed by average successive variability (ASV) or SD of all HbA 1c measurements after HF diagnosis. Multivariable Cox proportional hazards models were used to estimate the adjusted hazard ratio (HR) and its corresponding 95% CI. A total of 65 950 patients with HF were included in the study. Over a median follow-up of 6.7 (interquartile range, 4.0-10.6) years, 34 508 patients died and 52 446 required HF rehospitalization. Every unit increment of variability in HbA 1c was significantly associated with higher HF rehospitalization (HR ASV, 1.20 [95% CI, 1.18-1.23]) and all-cause death (HR ASV, 1.50 [95% CI, 1.47-1.53]). Diabetes significantly modified the association between HbA 1c variability and outcomes ( P interaction <0.001). HbA 1c variability in patients with HF without diabetes conferred a higher risk of rehospitalization (HR ASV, 1.92 [95% CI, 1.70-2.17] versus HR ASV, 1.19 [95% CI, 1.17-1.21]), and all-cause death (HR ASV, 3.90 [95% CI, 3.31-4.61] versus HR ASV, 1.47 [95% CI, 1.43-1.50] compared with patients with diabetes)., Conclusions: HbA 1c variability is significantly associated with greater risk of rehospitalization and all-cause death in patients with HF, irrespective of their diabetic status. These observations were more pronounced in patients with HF without diabetes.