Background: Heart failure (HF) and breast cancer are 2 of the leading causes of death in postmenopausal women. The temporal association between HF and breast cancer in postmenopausal women has not been described. Objectives: This study sought to examine the temporal association between HF and breast cancer. Methods: Postmenopausal women within the WHI (Women's Health Initiative) cohort were studied. All prevalent HF and prevalent breast cancer at enrollment were self-reported. Incident hospitalized HF and breast cancer diagnoses were adjudicated through 2017. Results: Among a cohort of 44,174 women (mean age 63 ± 7 years), 2,188 developed incident invasive breast cancer and 2,416 developed incident hospitalized HF over a median follow-up of 14 and 15 years, respectively. When compared with a breast cancer- and HF-free cohort, there was no association between prevalent HF and incident invasive breast cancer and similarly, there was no association between prevalent breast cancer and incident hospitalized HF. Across the entire cohort, the median survival after incident hospitalized HF was worse compared with an incident invasive breast cancer diagnosis (5 and 19 years, respectively). In women with incident invasive breast cancer, prevalent HF was associated with an increased risk of mortality (hazard ratio: 2.28; 95% confidence interval: 1.31 to 3.95). In women with incident hospitalized HF, prevalent breast cancer was associated with an increased risk of mortality (hazard ratio: 1.66; 95% confidence interval: 1.03 to 2.68). Cause of death after incident HF was different only in women with prevalent and interim breast cancer compared with those without prevalent and interim breast cancer. Conclusions: In postmenopausal women, prevalent HF was not associated with a higher incidence of breast cancer and vice versa. However, the presence of incident invasive breast cancer or incident HF in those with prevalent HF or prevalent breast cancer, respectively, was associated with increased mortality.